The mean FST value observed 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of linkage disequilibrium will be addressed in a separate publication (G. H. POCSON, K. A. MESA and R. G. BOUTI- LIER, unpublished results).