Frequencies of three codominant alleles at the transferrin (Tf) locus and four of the six possible genotypes in a population of Nuttall's cottontails Sylvilagus nuttallii on an 87-ha study area in central Oregon were determined for parental stocks, trappable offspring, and over-winter survivors in 1974 and 1975. One rare allele disappeared during winter 1974-75 and did not reappear during the study. The Tf genotype frequency shifted in favor of Tf-BB between parents and offspring in 1974, remained stable over winter, shifted in favor of Tf-BC (to near the original frequency) between parents and offspring in 1975 and remained stable over winter. Allele and genotype frequencies were significantly different between 1974 and 1975 offspring; differences in frequencies between other samples were not significant because of the small number of cottontails that survived to spring each year. We were unable to discount the possibility of non-random breeding being responsible for observed differences, but, because survival of juveniles was related to moisture available for plant growth (and presumably to the moisture content in forage) during the cottontail breeding season and because frequencies of Tf genotypes of the four litters produced each year seemed related to available moisture, we postulated that precipitation was the selective force responsible for shifts in allele and genotype frequencies. Although we were unable to ascertain the probability that the polymorphism was balanced, the stochastic precipitation pattern seemed adequate to prevent fixation of an allele by selection if the selective mechanism was as postulated.