A new member of the Neocallimasticaceae (Spizellomycetales), Neocallimastix hurleyensis sp. nov., is described. This species was isolated from the ovine rumen and has been referred to previously as Neocallimastix sp., strain R1. It has a classical monocentric type of life cycle, with polyflagellated zoospores, endogenous development of the zoosporangium, and death of the thallus following zoospore release. Although N. hurleyensis is similar to two other members of the genus, N. frontalis (the type species) and N. patriciarum, the zoospores possess 8-16 flagella, have no equatorial constriction, and exhibit segregation in the localization of intracellular organelles. A large organelle (presumptive hydrogenosome) is situated in the posterior portion of the cell, in close proximity to the kinetosomal apparatus. It is the size, location, and complexity of this organelle that explicitly distinguishes N. hurleyensis from other members of the genus.