SUMMARY: The colour variation of a strain of Fusarium oxysporum Schlecht ex Fr. was found to depend on the production of carotenoids and naphthoquinones. The carotenoids were only produced as a result of exposure to light, whereas the production of naphthoquinones, which were pH indicators, was mainly regulated by the carbon/nitrogen ratio of the medium. The morphogenetic effects of radiation and of carbon/nitrogen ratio were also considered. A low carbon/nitrogen ratio favoured chlamydospore formation and aerial mycelium, whereas ultraviolet radiation, and to a lesser extent light, promoted macrospore and selerotium production.