American chestnut trees were inoculated with pairs of strains of Endothia parasitica, and the rate (b) of the expansion of the area of cankers was measured. Hypovirulence (H) agents in 1 strain of the pair decreased b markedly if both strains were in the same vegetative compatibility (v-c) group. With increasing numbers of v-c alleles different between the pair, the decrease in b became less and less until the decrease became insignificant when different alleles at 5 v-c genes separated the pair of strains. Although various strains in different v-c groups were used, the results were consistent. French and Italian H agents had similar effects on b.