Malevich invented pure abstraction. Born in Kiev in 1878, he came to Moscow around 1908 and was drawn into the circles of young avantgarde artists around Mikhail Larionov and Natalia Goncharova. They were interested in the stark forms and bright uncompromising colour of Russian folk art, the French cubist movements after Paul Czanne and they were friends with the futurist literati and their leader, Vladimir Maiakovski. Designing a backdrop for the futurist play Victory Over The Sun in 1918, Malevich quite simply set a great big square densely filled with black paint plumb in the middle of a white cloth. A great black disc on white followed, then a cross on white, a White On White (now a jewel at MOMA), all in the 1920s. Gradually other geometric shapes, oblongs and squares, evenly painted in primary colours, and clear splinters and staves like exclamation points in black, swarmed and balanced harmoniously on wide-open white. Spare and sparkling, these flat paintings were the shields of a poetic defiance. It was Suprematism. As Malevich wrote: the experience of non-objectivitythe supremacy of pure feeling.

In the 1920s he also made idealised sculptural inventions of architecture in plaster calling them Arkitekton wrote theoretical books and became an influential teacher. But after 1929 his verve declined and his work became ordinary. After a series of peasant scenes close to his early cubist manner, he first made fake renaissance then fake French impressionist oils, and in the end made run-of-the-mill portraits of earnest stakhanovist Soviet worker heroes.

Whether this happened because he slavishly knuckled under to the demands of Stalinist realism, or whether the clear strength of his vision ran out, will be a mystery forever.

The selection here begins with early illuminist golden paintings and follows with cubist work. There are strange staring and whimsical figures on white in later days.

But the pioneer suprematist masterpieces are the fulcrum now the very icons of modern art; a handful of stern admonishing symbols of purity, the sparkling compositions of the 1920s. Of this time is a collage with the Mona Lisa crossed out, and the splendid fine Supremus No. 56, which are outstanding. This group and perhaps some addition from the peak of Malevichs suprematist period would have made a cleanly representative show. There was one like that which took place at Galleria Nazionale dArte Modena (GNAM) in 1959 and is fondly remembered by old-timers in Rome.

But in this exhibition, his best and typical work, his profound achievement, is obscured and confused by too many indifferent examples from early and late periods, and his fall from grace is made far too obvious. If this were not sad enough, the insensitive installation in old bank vaults with black walls and harsh spotlights makes the works look like desperately-valuable goods and not the bright witnesses of our cultural heritage they truly are.

Kazimir Malevich (1878-1935). Until 17 July.

Museo del Corso, Via del Corso 320. tel. 066786209.