Ascocotyle howei, n. sp., is described from adults found in the intestines of naturally infected Colombian opossums, Didelphis marsupialis. The new species is characterized by a distinctive flagon-shaped body, and 24-28 scalpel-shaped spines per row, arranged in two complete circumoral rows around a massive oral sucker. This new species most closely resembles A. megalocephala by way of a large oral sucker, a reduced pre-oral lip, a short ceca, and vitellaria which extend anteriorly to the level of the pharynx and posteriorly to the upper margin of the testes. Some characters, however, would place it in subgenus Ascocotyle or even Phagicola. In an introduction, the typical life histories and anatomical designations are reviewed for Ascocotyle trematodes, commonly referred to as flukes. Remarks concerning sibling speciation and its relevance to creation are summarized in the appendix.