Effective treatment recommendations for feline ringworm require accurate and reliable information on the effectiveness of antifungal therapies. The fungicidal activities of many products, particularly disinfectants, are determined by using the culture plate form of M. canis and not the naturally infective form (i.e., infected hairs and spores). Recent studies have shown, however, that the use of the naturally infective state of M. canis provides a more accurate assessment of a compound’s antifungal properties. The results of these studies suggested that there might be some variation in the susceptibility of different M. canis isolates to antifungal compounds. The purpose of this study is to determine if strain variation of M. canis exists. The existence of different strains of M. canis has great potential to affect how feline ringworm is treated. This study is the first step in determining whether or not routine diagnostic tests for culture and sensitivity testing of M. canis isolates will improve treatment recommendations for the disease.