A black aphid collected in Manitoba from several different host plants is so similar morphologically to Aphis fabae Scopoli that it has been given the name for all specimens collected. The true Aphis fabae is an economic pest, and thrives on faba bean, Vicia faba L. and other beans, in Europe and some parts of North America. When it was discovered that the Manitoba "Aphis fabae" would not live on faba bean, the present study was initiated to determine if it is (a) a physiological race or biotype of A.fabae, or (b) a subspecies of A.fabae, or (c) a species distinct from A.fabae. Live cultures of aphids were obtained from British Columbia and New Brunswick to compare with the Manitoba species. In field and laboratory experiments there were, from the three geographical areas, three cultures which would live on faba bean but not on nasturtium (regarded as the true Aphis fabae) and three cultures which would live on nasturtium, Tropaeolum majus L., but not on faba bean. A biosystematic approach was used to try to find differences which would be of specific rank, which included use of the scanning electron microscope, cytotaxonomy, physiological and ethological studies, and the conventional methods of counting and/or measuring anatomical features. On the basis of the following observations it was decided that the species in Manitoba is not Aphis fabae Scopoli:...