THE production of setae during the formation of the acervulus in the genus Colletotrichum has for some time been regarded as of doubtful value in distinguishing between the genera Colletotrichum and Gloeosporium. Schilling1 suggested that an investigation into the factors affecting seta production would help greatly in the solution of these taxonomic difficulties. The results presented here are part of a study of the whole process of asexual sporulation in Colletotrichum linicola Pethybr. and Laff., the causal organism of seedling blight of flax, both on the host plant and in pure culture.