We have examined the DNA content of spermatozoa from house mouse hybrids between ST 40 chromosome karyotype and Apennine populations with 22 chromosomes, as well as from hybrids between CD and CB strains both possessing 22 chromosomes, but characterized by dissimilar Robertsonian fusions. In the spermatozoa from 22 × ST 40 strains the variations in DNA content were found to fit the variability in gamete ploidy as estimated in spermatids by scoring second meiotic metaphase chromosomes. This demonstrates that during spermiohistogenesis aneuploid spermatids are not eliminated. The rare spermatozoa produced by CD×CB hybrids display a wide variability in their DNA content, with a low percentage of values referable to haploid cells.