CHLAMYDOSPORE formation and their numbers have been found to vary between species in the genus Fusarium and have been widely used in systematic nomenclature as a key character1,2. Nevertheless, no positive evidence has so far been adduced on the causes inducing the formation of this morphological structure, although it is generally presumed to be the result of unfavourable growth conditions. Control of soil-borne infection of various crop plants has been achieved by the addition of specific antibiotics3 or by suspensions of soil bacteria and their toxins4–6. These observations, together with the more recent discovery7 of plants absorbing antibiotics produced by soil micro-organisms, suggested the probability of this factor stimulating chlamydospore formation in Fusarium mycelia.