The processes of spermatogenesis and spermiogenesis in Hymenolepis diminuta were studied by electron microscopy using improved preparative techniques. Spermatogonia (Type A) are characterized by nuclei 3.79 (+/- 0.17) micrometer in diameter, dense cytoplasm packed with free ribosomes and aggregates of mitochondria. After mitoses, certain spermatogonia (Type B) assume syncytial rosettes containing eight nuclei. Primary spermatocytes maintain the rosette syncytium and have large nuclei (4.28 +/- 0.24 micrometer in diameter), smooth endoplasmic reticulum, and polysomes. The secondary spermatocyte is short-lived and is characterized by nuclei (2.0 +/- 0.11 micrometer in diai (2.0 +/- 0.11 micrometer in diameter) and perinuclear membranous lamellae. The syncytial spermatid cluster contains avoid nuclei which condense and elongate to a final diameter of 0.22 +/- 0.04 micrometer. Once elongated, these nuclei become delimited from the syncytium by invaginations of the plasma membrane. During delimitation, cortical peripheral microtubules arise beneath the spermatozoon plasmalemma and a 9 + 1 axoneme extends the length of the mature lance-shaped spermatozoon.