Two geographic founder colonies of Cotesia flavipes from Pakistan were imported for biological control of Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae) and other African cereal stem borers. Separate colonies were initiated from the two importations and examined for electrophoretic variation at 14 isozyme loci over 22 generations of laboratory rearing. Two out of 14 loci examined were found to be polymorphic. In the first 12 generations of sampling, when approximately 1000 breeding females were used to perpetuate both colonies, there was a slight increase in heterozygosity at the MDH locus in the colonies of the two strains. In the last 10 generations of sampling, when only 500 females were used, there was a rapid decrease in electrophoretic variation in the two colonies. The effective population sizes for the two colonies was estimated at 3.4 and 9.0% of the number of breeding females used to continue the colonies. These data indicate that using 1000 mated females to perpetuate the C. flavipes colonies maintained greater genetic variation than using 500 females.