Apterous apple aphids (Aphis pomi de Geer) collected from several southern Ontario orchards in several seasons were studied electrophoretically for 17 enzyme and 2 protein loci. Sixteen loci proved monomorphic. The remaining three showed a strong association among loci; all individuals that are homozygous for the fast allele of glutamate oxaloacetate transaminase (Got) also have no esterase-1 activity (presumably being fixed for a null allele) and have a unique banding phenotype for esterase-2 (a locus for which no Mendelian model could be made). The remaining aphids are fixed for the slow Got allele, have two active alleles segregating at esterase-1 and show either of two banding phenotypes at esterase-2. Apparently. Aphis pomi consists of two differentiated, noninterbreeding subpopulations. The first type has increased from rarity to near fixation in southern Ontario since 1978, and may be spreading up the Niagara Peninsula. It has greater resistance to malathion than the replaced type.Key words: aphid, allozymes, population genetics, polymorphisms, speciation.