Bird, J., B. Riska, and R. R. Sokal. (Department of Ecology and Evolution, State University of New York at Stony Brook, Stony Brook, New York 11794) 1981. Geographic variation in variability of Pemphigus populicaulis. Syst. Zool., 30:58-70.-Geographic variation of variability profiles is studied based on stem mother, alate and gall characters of the aphid Pemphigus populicaulis. Significant differences among the coefficients of variation of the suite of characters in every locality were found by two methods-Bartlett's test for homogeneity of variances (using log-transformed data) and Friedman's nonparametric test for randomize'd blocks. Nine characters are highly variable and six are consistently low in variability. Coefficients of variation show significant heterogeneity among localities for every character by Bartlett's test, and for all but two characters using a modified Friedman's test. The variability profiles of the 66 localities were clustered into three groups based on both size and shape by the k-means algorithm. A mixture of two types of profiles occurs throughout the middle latitudes of the study area, while the third type is found in its northern and southern regions. This geographic pattern may in part be due to the year of collection. Attempts to relate the magnitude of the coefficients of variation to several ecological indices were unsuccessful. [Variability, variability profiles, geographic variation, coefficients of variation, Pemphigus.] This study investigates whether relative morphological variability varies geographically. A measure of variability is a fundamental parameter in evolutionary research because of the intimate relationship between natural selection and variability. Natural selection shapes variability and is in turn limited in its potential effects by the extent of the variability of a given character. Low variability in a