Analysis of microspread, silver-stained primary spermatocytes from chromosomally and phenotypically normal Peromyscus sitkensis revealed the occurrence of XYY zygotene and pachytene nuclei at low frequency in three of eight individuals examined. Observed pairing configurations of sex chromosomes included a trivalent and a Y bivalent-X univalent. The data suggest that premeiotic nondisjunction may be involved in the origination of XYY chromosomal conditions.