A laboratory method developed by J. Grente was used to test the abilities of 7 related hypovirulent strains of E. parasitica to convert 49 virulent vegetative compatibility (v-c) tester strains to hypovirulence. The success of conversion, its rate, and the stability of the converts are all influenced by vegetative compatibility. Rapid conversions (2 days or less) always occurred between strains of the same v-c group and were common in certain incompatible pairings. In other incompatible pairings, conversion was slow and erratic, and 10 of the testers were not converted. Examination of ds[double strand]RNA components in 2 series of replicated incompatible rapid conversions showed that variation and stability of the dsRNA patterns also were influenced by the v-c group of the recipient strains. Mechanisms for maintaining stable dsRNA genomes and for generating variants of these genomes contribute to the polymorphism and adaptability of this biological control agent.