Isolates of Verticillium lecanii, from insects and other substrates, were tested for vegetative compatibility by observing heterokaryon formation among complementary nitrate-nonutilizing (nit) mutants, Among 33 V. lecanii isolates, 17 were self-incompatible. The 16 self-compatible isolates were divided into 13 vegetative compatibility groups (VCGs). Eleven VCGs were single-member VCGs, and the remaining two included two or three isolates each. Self-incompatibility occurred more frequently among isolates with abundant aerial mycelium, whereas most of the isolates with short or reduced aerial mycelium were self-compatible. Virulence to larvae of Bemisia tabaci ranged from 0 (four isolates) to 83% mortality 4 d after treatment with conidial suspensions. There was no correlation between the capacity of isolates to anastomose and their virulence to larvae of Bemisia tabaci: highly virulent isolates were found at similar frequencies among self-compatible and self-incompatible isolates. Those with abundant aerial mycelium were collectively more virulent (28·5% mortality) than those with reduced mycelium (10·7% mortality). Three isolates belong to VCG VL-11 were all highly virulent.