In an earlier study (Rhombergh et al., Can. J. Genet. Cytol. 27: 224-232, 1985) of natural populations of the cyclic parthenogenetic Rose aphids, Macrosiphum rosae, 6 out of 31 loci were found to be polymorphic and one locus (Esterase-4) showed cyclic seasonal changes in gene and genotypic frequencies. Assuming that the Est-4 polymorphism was balanced and due to some climatic factor that varies seasonally, and realizing that most environmental factors that vary seasonally also vary latitudinally, we predicted existence of a latitudinal gene cline at this locus. In the present study we surveyed four polymorphic loci (chosen to be used as markers) in six geographic populations spanning over 1200 km between the United States and Canada and found all four loci to have latitudinal clines. We think that the gene clines are due to a latitudinal cline in the degree of advancement of local populations through the seasonal cycle, and have called such a pattern a 'seasonal phase cline'. The results are discussed in relation to the temporal instability of local patterns and persistence of genetic variability on the large scale in aphids. It is argued that population structure of aphids makes retention of selectively neutral or weakly selected polymorphisms difficult.