The effects of spermatogenesis of a series of continguous non-overlapping Y-chromosome deficiencies were examined using both the light and electron microscope. The deficiencies were constructed by combining elements of different X-Y translocations; they subdivide the Y into seven segments, six of which are required for male fertility (four in the long arm and two in the short arm). Spermatogenesis was examined from the primary spermatocyte through to the formation of mature sperm and the earliest departures from normal development identified. Two deficiencies result in the absence of the same structure from the axoneme of the sperm tail--the dynein-containing outer arm extending from the A subtubule of the peripheral doublet; they also result in the absence from primary spermatocyte nuclei of aggregates of tubuli in one case and reticular material in the other. A third deficiency causes the appearance in the primary spermatocyte of the crystals characteristic of X0 males and the irregular distribution during meiosis of nuclear and cytoplasmic elements to the spermatids. The fourth deficiency results in the misalignment of the developing axoneme with the mitochondrial derivatives and is first detectable in the onion nebenkern stage of the spermatid. Finally for two deficiencies the first abnormalities detected were during later stages and comprise a syndrome found in most of the steriles. We attribute this phenotype to the indirect effects of earlier lesions.