Indirect estimates of differential early mortality at the transferrin (Tf) and leucine amino-peptidase (LAP) loci in fluctuating populations of Microtus pennsylvanicus have been obtained. Among individuals first captured during decline phases, there were consistent excesses of males heterozygous at the Tf locus. Among individuals first captured during increase phases, there were consistent excesses of males heterozygous at the LAP locus. It is concluded that these observations are attributable to selection.