Lemmings are a classic example of animals with strong population cycles. High-amplitude density fluctuations with low numbers during the low phase are expected to lead to strong genetic drift, which erodes genetic variability. By compiling data on mitochondrial DNA polymorphism for 72 lemming populations from 5 species, we found no evidence for this erosive mechanism. On the contrary, high levels of haplotype diversity (average h of 0.75 for samples of the genus Lemmus) were observed in many populations. Although the effective size determines the level of genetic diversity in closed populations, diversity is primarily determined by immigration in open populations. Simulations of genetic drift in open populations fluctuating in density confirmed the independence of genetic variability from local effective size, and predicted a deficit of rare alleles, as observed in lemming samples. High genetic variability thus implies high gene flow over a considerable area for lemmings, but does not provide information about the local effective size of populations. Examination of empirical data suggests that high genetic diversity may be the rule rather than the exception in cyclic populations.