Mature, umated male and female Brugia pahangi worms were implanted into the peritoneal cavity of gerbils, allowed to mate, and then recovered and examined by means of electron microscopy. The proximal portion of the uterus of female worms recovered in copula contained a morphologically heterogenous population of sperm ranging from a rigid, nonmotile form to the mature ameboid sperm. The immotile sperm are identical in morphology to sperm found within the seminal vesicle of the male. The in utero transformations occur in less than 1 hr, and probably represent a final maturation event preceding competency to fertilize eggs. First in the sequence of events leading to the mature spermatozoon is a decrease in the density of the cytoplasmic matrix concomitant with the elaboration of an abundant tubular smooth endoplasmic reticulum. The position and appearance of the sperm in the uterus suggest that the formation of this membranous system occurs within minutes and involves de novo assembly from cytoplasmic precursors. Following the appearance of the tubular membranous system, certain other membranous organelles become spherical, fuse with the plasmalemma, and release their electron-dense contents into the lumen of the uterus. Filamentous rodlike elements, previously extending the length of the peripheral cytoplasm, begin to disintegrate and cytoplasmic projections form at the margins of the cell. Polarization of the sperm results from the aggregation of mitochondria, nuclear material, endoplasmic reticulum, and membranous organelles at one pole of the cell leaving an organelle-free filamentous pseudopod at the other.