Ancylostoma caninum spermatogonia appeared as unspecialized cells with large nuclei and little cytoplasm when examined by electron microscopy. Mitochondria and ribosomes were numerous but endoplasmic reticulum of either the rough or smooth variety was rarely observed. Microtubules were abundant but were mostly concentrated in areas where the cells connected with a central cytoplasmic core, or rachis. Microtubules present elsewhere in the cytoplasm were not associated with any particular organelle. In spermatocytes, microtubules became numerous and formed the spindle apparatus during the rapid meiotic divisions. Following the meiotic divisions the nuclei of the resulting spermatids were devoid of a nuclear envelope, and the chromatin became highly condensed. At this time, the microtubules surrounded the elongating nucleus as it became confined to the tapering, posterior portion of the spermatid. These observations indicate that microtubules are involved in determining this final morphological appearance of Ancylostoma spermatids. However, there is no evidence to indicate the microtubules are assembled into motile organelles to be utilized by the ameboid in utero spermatozoa.