Based on material from two stickleback species living in the same biotope,Proteocephalus filicollis(Rudolphi, 1802) andProteocephalus ambiguus(Dujardin, 1845) are regarded as two distinct species. The most useful character for their discrimination is the presence and absence of an apical scolex ofP. filicollisandP. ambiguus, respectively. Furthermore, transverse sections revealed that the longitudinal muscles consist of approximately 5 layers of muscle fibres inP. filicollis, as opposed to a single layer inP. ambiguus. The two species differ also in total length, width, number of segments and in the state (progress) of maturation of the proglottids along the strobila.