A biological expedition of the University of Ghent, Belgium, to Morocco, collected a great number of bivalves in the upper course of the Oued Mikkés in the Moyen Atlas mountains, at an altitude of about 1700 meters at the end of July 1971. The locality, near the Cascade des Vierges, is situated at a distance of 8 km from Ifrane on the road to Meknès. The material contains seven species of the genus Pisidium (Lamellibranchia, Sphaeriidae), viz., P. amnicum (Müller), P. casertanum (Poli), P. milium Held, P. nitidum Jenyns, P. personation Malm, P. subruncatum Malm, P. tenuilineatum Stelfox. The last five mentioned are new to the fauna of Morocco, P. nitidum and P. tenuilineatum even to that of Africa. Most surprising in this locality is the presence of P. tenuilineatum, firstly because never before has a representative of the Eurasian subgenus Odhneripisidium Kuiper been recorded on the African continent, secondly because elsewhere this species only lives below 500 m. Synecologically interesting is the comparatively large number of seven associated species of Pisidium, which is once more an illustration of the fact that the area under discussion faunistically belongs to the Palearctic Region. Finally, a number of comparatively large specimens of P. subtruncatum and P. tenuilineatum, and three types of inverted hinges (a, total inversion; b, inversion of both cardinals and anterior laterals; c, inversion of posterior laterals only) are worth mentioning from a morphological point of view.