Apple grain and English grain aphids collected in New York and Washington were compared as vectors of 28 collections of the Washington barley yellow dwarf virus, Aphids of each colony transmitted each of the 28 collections, for the most part, have little or no vector specificity in regard to tose of Washington, beging in no way distinguished in these test. These observations with those of Rechow (4) indicate that the prevalence of "vector-specific" strains in New York and their relative absence in Washington is due to a regional difference in the virus complex. The predominance of strains of BYDV adapted for transmission by the English grain aphid in New York and the relative absence of "vector-specific" strains in Washington may reflect a difference in aphid species dynamics. The hypothesis is advanced that the English grain aphid may have predominated for years in New York, but aphid species may have fluctuated abruptly and markedly in Washington. This latter situation would place selection pressure on the virus toward versatility in vector relationships.

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