This pattern is likely to reflect the action of drift. In contrast, RFLP locus GM798 detected highly significant allele frequency differences among all populations at a level exceeding that expected by chance alone. The FST val- ues calculated for the 11 independent cDNA loci are expected to be distributed as a chi-square with 10 de- grees of freedom. The mean FST valueo bserved for GM798 (0.309) exceeds the 95% confidence interval of this distribution, thus making drift an unlikely explana- tion for the heterogeneity observed. This suggests the possibility that the three polymorphisms scored by this cDNA clone may be tightly linked to a site undergoing selection. Further indirect evidence for the operation of selection at this locus based on geographic patterns of linkage disequilibrium will be addressed in a separate publication (G. H. POCSON, K. A. MESA and R. G. BOUTI- LIER, unpublished results).