The dysgonic strain of Microsporum canis is described. The colony consists of fascicles of very wide hyphae with short compartments and close branching. Lateral branches often themselves remain short and fail to branch, and many are reflexive, growing in the opposite direction to the parent hypha. Mycelium of three types was noted, dysgonic, apparently normal, and forms intermediate between the two. Change from one form to another occurred in a cyclial manner, the transition from dysgonic to apparently normal being abrupt while that in the reverse direction was gradual. The strain was extremely unstable, both in primary isolations from cats and in subculture. Numerous sectors of other readily recognisable strains were produced, including the typical M. canis and various atypical strains previously obtained from human sources. These strains rapidly submerged their parent colonies and were much more likely to be obtained by subculture from a primary isolation plate than was the dysgonic strain itself. The relationships of the various strains to each other in the saprophytic and parasitic phases are discussed.