Mutagenized avirulent mutants (AV) of Geotrichum candidum and mutants of reduced virulence had lower polygalacturonase (PG), cellulase (Cx), and phosphatidase (PT) activity values than the virulent wild-type strain (G.c. met-1). Some of the AV mutants yielded reduced virulence revertants after mutagenesis. These revertants showed a substantial increase in PG, Cx, and PT activity but did not equal that of the wild-type strain Activities of PG, Cx, PT, and pectin methylesterase (PME) in rotted fruit were much higher than those in healthy fruit In vitro, G. candidum produced PG, Cx, and PT but no PME, pectic acid transeliminase, or pectin transeliminase. No heterokaryons were produced for complementing auxotrophic mutants of G. candidum or of Endomyces geotrichum or between species.