The equine parasites which have so far been classified as belonging to the single species Parascaris equorum (Goeze) have been shown to belong to two distinct species on the basis of morphological and karyological characters, and of the electrophoretic study of 27 enzyme loci. The name Parascaris univalens is proposed for the species with distally truncated spicula and karyotype 2 n = 2. The name “univalens” was used by Hertwig in 1890 to indicate horse ascarids with this karyotype. A complete description of the species is given. The name Parascaris equorum is maintained for the other species with distally rounded spicula and karyotype 2 n = 4 (corresponding to Hertwig's “bivalens” form) because Yorke and Maplestone, in their work describing the genus Parascaris, also drew the caudal extremity of the male of the type species P. equorum with distally rounded spicula.