Phacopid trilobites are relatively widespread in Devonian deposits of north Gondwana and some have been collected from several sections of the Saoura Valley in SW Algeria. New occurrences of phacopids assigned to Trimerocephalus, Dianops and Phacops sensu lato are described from this area and comparisons are made with closely allied species from Morocco and Europe. The trilobite assemblages of the ‘Argiles de Marhouma’ Formation are considered to be Frasnian–Famennian in age. These new occurrences have been integrated into an analysis of Upper Devonian phacopid biodiversity. Diversity fluctuations reflect environmental changes, bioevents and stratigraphic turnovers throughout the Upper Devonian. Peak diversity was attained after the post-Kellwasser event.