The site specificity and developmental morphology of Oligacanthorhynchus microcephalus was studied in its definitive host the Virginia opossum (Didelphis virginiana). Opossums were infected with cystacanths removed from the hemocoels of the millipede Narceus americanus, the intermediate host of O. microcephalus. At 2, 4, 6, and 8 wk postinfection (PI), an opossum was killed and the small intestine was removed and divided into 20 equal sections. The locations of all worms were recorded and worms present used to describe morphology at each time interval PI. Collectively, worms occurred in intestinal segments 2–15. No worm occurred in the first intestinal segment or in the last five segments of the intestine. Worms demonstrated a moderate degree of site specificity, with the majority of worms aggregating between intestinal segments 9 and 13, accounting for 69% of the total worm burden. Commencing at 2 wk PI, growth rates of females were almost twice those of males. Although the growth rate of both sexes slowed over time, both sexes continued to grow after achieving sexual maturity. Cement caps were found on females collected at 4 wk PI, 6 wk PI, and in adult worms, indicating that copulation commences between 2 and 4 wk PI. Proboscis length and width did not change throughout development. In males, testes increased in size and became separated by progressively increasing distances throughout development, although they remained within the posterior third of the trunk.