In Ostend, the social season reaches its peak with annual celebrations in honour of a dead rat. Every year this eminently respectable seaside resort throws dignity to the winds at carnival time, when some 3,000 people crowd eagerly into the local casino for Le Bal du Rat Mort and the licence of the masked charade spills boisterously over into the streets.

The story goes that the cult of the defunct animal derives from the painter James Ensor, who frequented a hostelry of the same name in Paris during his younger days. It was the favoured haunt of burlesque girls, artists, assorted weirdoes and knees-up brigades. Ensor, whose taste for the bizarre developed at an early age, decided that this was just what his home town needed. Returning to Ostend, he started a series of parties for friends garbed in grostesque masks and costumes, and within a few years these became so popular that the municipality formed La Compagnie du Rat Mort to promote the bal as a civic event - and incidentally pay Ensor the sort of tribute he enjoyed.

He is, after all, the town's most famous son, and the massive retrospective exhibition now marking the 50th anniversary of his death at the Musees Royaux in Brussels makes it clear that, while his imagination ranged far beyond the norm, throughout his life his principal source of inspiration remained the landscape in which he lived, and above all, the images and artefacts of his childhood.

A dead rat must seem commonplace to a man whose grandmother's constant companion was an ape dressed up in outlandish costumes, as her grandson was from time to time; and whose delight in masquerade prompted her to scare the fiveyear-old witless by appearing at his bedside in a hideous mask.

This extraordinary woman kept a curio shop, selling seashells, lace, stuffed fish, old prints, books and weapons, and the memory of it informed much of Ensor's work. "This confused mass of eccentric objects," he recalled, "was constantly being knocked over by one of several cats, squawking parrots and a monkey . . . The shop smelled musty, the seashells were full of pungent monkey wee, and the cats would snuggle down among the most precious pieces of lace."

As his parents were in the same business, it is hardly surprising that the family stock-in-trade featured prominently in Ensor's art. But they were bourgeois too, taking a lively interest in food, and an arresting still life from Ensor's 20s is a brilliant study of the most luxuriant green cabbage imaginable.

Although their origin is obvious, the masks are something else again. For Ensor, their repellent aspect provided a means of mocking the things that frightened him, and he expressed his feelings through the medium of the explosively bright - almost giddy, colours that were a characteristic of his maturity. The masks were often accompanied or worn by skeletons, a singularly ghoulish crew in Ensor's concept, though in fact the most striking of the whole collection is a relatively early painting of Skeletons Trying to Warm Themselves. They are not wearing masks, though one of them sports a top hat, and a strong sense of pathos emanates from the poor figures, some of them recumbent on bare floorboards, others pressing forward towards an old-fashioned stove.

James Ensor was born in 1860 and began drawing when he was 13. In 1877 he was admitted to the Brussels Royal Academy of Fine Arts, but did not do well there. Rejecting formal academic teaching, after three years he went back to Ostend to live with his parents again, and made the town his home for the rest of his life.

The years that followed were prolific, and this period, with its telling studies of cluttered salon interiors, marks the onset of his preoccupation with light. Gradually, as his expertise developed, this became a component of his style, but it remains at its most isolated and dramatic in the early phase, represented by the painting of two nude children dressing against a background of blazing colour.

Like many another artist, Ensor met with plenty of reverses and rejection before achieving the distinction that earned him decorations at the hands of two successive kings of Belgium. Owning a good conceit of himself, as his numerous self-portraits indicate, he was not one to forgive, and those who offended were mercilessly pilloried. The best example of this must be The Dangerous Cooks, which shows two art critics of the time preparing the heads of Ensor and some friends for the edible delectation of the cooks' colleagues seated in willing anticipation at a table in the background.

There are several other pictures in similar scathing vein, including The Bad Doctors, which caricatures members of the medical faculty at Brussels University, and a hilarious send-up of the conservatory in Brussels. This last is dominated by a portrait of Wagner forcing his fingers into his ears, while below a well-known conductor of the day directs a motley gang of musicians headed by a soprano belting out the opening notes of the Valkyries' cry.

However, Ensor's humour was not always cruel, and there is no denying the light-hearted charm of the countless comic figures in The Baths at Ostend or the skaters slipping and tumbling on the ice in a superb copper engraving of 1889.

The same is true of Ensor's designs for his own light-music ballet, La Gamme d'Amour, but on the whole, the darker side of his nature prevailed, and becomes particularly evident in his treatment of religious subjects.

Only a few - notably The Virgin of Consolation, which depicts the painter kneeling before the Madonna - illustrate the masterly and beautiful simplicity that was among his gifts. Most are mordant and astonishingly complex in composition, and while dimly lit for the sake of preservation, the drawings and etchings which form an important section of the Brussels exhibition reward painstaking scrutiny for the unexpected revelation of the artist's compassion for humanity, - all too frequently scarred by sardonic mockery.

THE etching of Death Pursuing the Human Flock is to my mind the most powerful, but of course Christ's Entry into Brussels is the more celebrated and by far the more shocking. Here, with innate effrontery, Ensor substitutes his own head for that of Christ, peoples the surroundings with hordes of tiny figures, many masked, and emphasises the secular ambience of the occasion with commercial and political banners, the most blatant of them an advert for Colman's Mustard.

Widely acknowledged as Ensor's masterpiece, this amazing picture, which dates from 1889, is now the property of the Getty Museum in Malibu, and fearing damage in transit, the museum refused to loan it for the exhibition, leaving visitors to content themselves with a fine print made the same year, and some later etchings.