Ecoepidemiological analysis of a Moroccan focus of leishmaniasis caused by Leishmania tropica revealed considerable enzymatic diversity. Seven zymodemes belonging to the complex were identified in 149 strains isolated from humans, dogs, and the vector Phlebotomus sergenti. Three distinct subgroups were identifiable, two of which were in turn, composed of three 'small variant' zymodemes. The diversity appears to be related to the age of the focus, which may have allowed colonization by zymodemes of different geographic origins. Diversification into 'small variants' is apparently the result of recent mutation, possibly associated with genetic exchange.