Previous investigations in this laboratory of heterokaryosis in T. cucumeris have utilized mutant progeny of one field isolate. The work has now been extended to include four field isolates, of different geographic origin, yet of similar host specificity in their pathogenic reactions (viz. all pathogenic to crucifers only among the hosts tested). Mutant single.spore cultures derived from these four have been induced to form inter-isolate heterokaryons, regardless of their differences (e.g. in origin, cultural appearance, fruiting ability, survival ability). Complementation for pathogenicity and fruiting behaviour was observed in these heterokaryons and wild-type progeny showed no segregation in pathogenic reactions. This suggests genetic similarity between all four isolates in regard to the factors controlling pathogenicity. However, wild-type progeny exhibited considerable variation in cultural appearance, the extent varying with the mutants used. Cultural differences may be a reflection of different saprophytic abilities.