The chromosome constitutions of sperm selected for motility according to the swim-up technique were compared cytogenetically with those of sperm remaining in the semen with the use of the human sperm/hamster egg system, in which human sperm are fused with hamster eggs to give analyzable haploid chromosome complements. Three semen samples from one donor resulted in 153 chromosome complements from selected, highly motile sperm and 110 unselected, control complements. Four samples were donated by another man, from which 181 selected and 186 control complements were obtained. The frequencies of chromosomal aberrations recovered from the population selected for high motility and the unselected population were not statistically different from one another.