The frequencies of cells with XY nonpairing at diakinesis/metaphase I were examined in random-bred ICR-Swiss, partly inbred DCH, and highly inbred DBA/2J and C57BL/6-bgJmice. The animals in a given inbred line displayed homogeneity in the proportion of cells with gonosomal univalents. In DBA/2J, the X and Y remained unpaired in 33.2% of the cells examined, whereas the corresponding frequency in C57BL/6-bgJ was only 7.0%. Previous data of Rapp et al. (1977) showed a low incidence of XY non-association in C57BL/6 and high frequencies in the BDF, hybrid (C57BL6 ♀ × DBA/2 ♂). We interpret our results, and the previous data of Rapp et al. (1977), to indicate the existence in mice of a genetic polymorphism for XY nonassociation. The DBA/2J strain appears to have factors that predispose to a high frequency of gonosomal unival