seventeen clones of rhopalosiphum padi (linnaeus, 1758), 14 of r. maidis (fitch, 1856), and two of r. insertum (walker, 1849), representing a wide range of host plants and geographic distribution, were examined electrophoretically to determine intra and interspecific variation. twenty-one enzyme systems were tested using starch-gel techniques. the electromorph variation within species was low, as expected for parthenogenetic organisms. frequency of heterozygotes was also relatively low for most populations. the percentage of polymorphic loci ranged from 0% to 27.3% in r. padi, but it was lower (0% to 18.2%) in the completely an holocyclic r. maidis. no consistent correlation between band patterns and host plant or geographic origin could be established for r. padi and r. maidis. the distinction between r. padi and r. insertum was made by est-1, lap-2, pgm, got-1,and α-gpdh.the enzymes est-2, g-3pdh, sdh, and got-2 were useful to separate r. maidis from r. padi, while est-1, lap-2, got-1, and α-gpdh, in addition to the first three mentioned above, separated r. maidis from r. insertum. however, many other loci shared only one electromorph and had another that was useful to partially distinguish the species. the utilization of starch-gel electrophoresis as a taxonomic tool in distinguishing populations and species of aphids and its combination with other taxonomic methods are briefly discussed.