Nine microsatellite loci were developed and assayed for polymorphism in four groups of the insect parasitoid, Cotesia congregata (Say) (Hymenoptera). One locus was monomorphic and eight were polymorphic in C. congregata. Allelic distributions differed significantly between groups of C. congregata collected from solanaceous and catalpa host food plants (P = 0). Loci were tested for cross-reactivity to six congeners: C. acuminata (Reinhard), C. bignellii (Marshall), C. cynthiae (Nixon), C. euphydryidis (Musebeck), C. koebelei (Riley) and C. melitaearum (Wilkinson). In the congeners, eight loci were polymorphic, four of which yielded allele sizes that differed substantially from sizes of amplification products observed for C. congregata.