The ultrastructure of the head end and surface structure of the cuticle ofSyphacia petrusewiczi, S. nigeriana, S. fredericiandS. stromawas studied. These species may be easily separated on the basis of the differences in their morphology:S. fredericipossesses longitudinal septa on the body surface, a row of denticles on each of the three main teeth, and cervical alae;S. nigerianahas longitudinal septa and denticles but lack cervical alae; S.petrusewiczihas longitudinal septa and cervical alae but lacks denticles;S. stromalacks these three characters.