The host preferences and sex ratios of Mesochorus biotypes from Sweden and Colorado (U.S.A.) were studied in the field and laboratory. These Mesochorus are secondary parasites of Bathyplectes spp., parasites of Hypera postica (Gyllenhal). The behavioral differences and reproductive isolation of the thelytokous and arrhenotokous biotypes indicate that they are separate sibling species, even though they cannot yet be separated morphologically, and isozyme analyses were inconclusive (possibly due to a mixture of biotypes in Sweden). none