A well-preserved and moderately diverse organic-walled microphytoplankton assemblage was recovered from the subsurface Lower Ordovician Nambeet Formation, Canning Basin, Western Australia. The microphytoplankton assemblage consists of prasinophyte phycomata (Leiosphaeridia spp.), acritarchs, cyanobacteria (Eomerismopedia maureeniae), degraded algae, chitinozoans and graptolite fragments. The acritarch assemblage comprises 13 genera, one of which is new (Aciculasphaera), 13 named species, three of which are new (Aciculasphaera interrupta, Gorgonisphaeridium martiniae and Loeblichia nambeetense), five species left in open nomenclature (sp.), and one cf. designation. The acritarch assemblage indicates an Early Ordovician (late Tremodocian through Floian) age and displays some similarities with comparable-age assemblages reported from North and South China, Australia and Laurentia. The paleogeographic distribution of the acritarch assemblage indicates that this assemblage represents a low- to mid-paleolatitude occurrence. Sedimentologic and palynologic evidence signifies deposition in a normal marine offshore environment.