Sermatozoa from two brothers who are not twins were found to be straight and immotile. Examinations of the sperm showed that oxygen consumption and lactic acid production were normal; viability tests showed that the percentage of dead sperm was not increased. The ultrastructural appearance of the sperm tail was normal except for a complete lack of dynein arms and some irregularities in the arrangement of the accessory fibers and the longitudinal columns of the fibrous sheath. The mitochondrial apparatus and the sperm head conform to the conventional model. According to the sliding-filament hypothesis first proposed by Afzelius (1959. J. Biophys. Biochem. Cytol. 5:269.), the arms are responsible for the bending movements of the tail. The simplest explanation for the simultaneous lack of arms and sperm motility appears to be that the two brothers have a genetic disorder involving production, assembly, or attachment of the dynein arms.