Samples ofRhopalosiphum maidis(Fitch) taken from corn in Idaho and Montana from 1985 to 1987 were karyotyped and found to be 2n = 8, whereas those from barley and barnyard grass (Echinochloa crus-galli(L.) P. Beauv.) were 2n = 10. Samples from wheat in Idaho in 1987 had a mixture of karyotypes (2n = 8, 9, and 10). The results indicate highly selective colonization of corn and barley by different genotypes ofR. maidis, so it is unlikely that this aphid can carry barley yellow dwarf virus (BYDV) between these two crops.E. crus-gallimay be an important reservoir host for BYDV isolates vectored byR. maidisto barley and winter wheat.