Bivalve molluscs feeding on toxigenic Pseudo-nitzschia spp. are the main vector of domoic acid (DA) to humans. Although different oyster species rarely exceed the internationally adopted regulatory level for shellfish harvesting closures (20 ub5g gu20131), these are often applied to all bivalve species in an affected area. This study examines the influence of diet composition and Pseudo-nitzschia multiseries cell size, density and toxicity on oyster feeding rates to determine potential pre-ingestive mechanisms that may lead to low DA accumulation in the oysters Crassostrea virginica. Clearance rate (CR) and pseudofeces production of juvenile oysters were quantified under varying laboratory conditions representative of natural Pseudo-nitzschia blooms. Oysters filtered increasing amounts of P. multiseries cells as cell density increased, but ingestion was limited by pseudofeces production at concentrations >10400 cells mlu20131 (ca. 6 mg dry weight lu20131). Oysters significantly reduced their CR when fed both toxic and non-toxic P. multiseries clones in unialgal suspensions compared to Isochrysis galbana, and this rapid grazing inhibition was not related to growth stage, cell size, or exposure time. When offered mixed suspensions containing equivalent cellular volumes of the 2 species, however, relatively high CR was restored. Therefore, we suggest that DA intake by oysters from mono-specific Pseudo-nitzschia blooms would be limited by a persistently reduced CR and rejection of cells in pseudofeces. When an alternative, good source of food is present in a mixed phytoplankton assemblage with P. multiseries, no CR inhibition is expected and DA intake will be regulated by pre-ingestive particle selection on the feeding organs, as demonstrated in Mafra et al.Peer reviewed: YesNRC publication: Ye