Bivalve molluscs can pose a public health hazard by accumulating high levels of domoic acid (DA) when suspension-feeding on toxic Pseudo-nitzschia spp. DA has also caused mortalities of marine fauna and economic losses due to shellfish closures worldwide. Although there are no field reports to date of DA contamination above the regulatory limit (20 Âµg.g-1) in oysters, harvest closures are often imposed on all bivalve species, as occurred during an anomalous early spring bloom of P. seriata in the southern Gulf of St. Lawrence in 2002. The present study a) compares DA uptake rates in Eastern oysters (Crassostrea virginica) and blue mussels (Mytilus edulis) under common laboratory conditions, and b) investigates feeding responses in oysters that may explain their relatively low capacity for DA accumulation. Our findings will be used to evaluate the potential for species-specific management of DA-contaminated shellfish. In two long-term trials (~2 wks), juvenile oysters and mussels were offered either P. multiseries clone CLN-20 (24 Âµm cell length) or clone CLN-50 (81 Âµm) at a constant density (8,500 and 1,800 cells.ml-1, respectively), in stationary growth phase, at 12ÂºC. These clones were almost exclusively singlecelled. Pooled individuals were sampled in triplicate over time to determine DA content in tissues by HPLC. Short-term feeding experiments were also conducted to determine clearance rate (CR) and pseudofeces production of oysters fed toxic P. multiseries clones of various sizes. They were offered as unialgal diets or mixed with an equivalent cell volume of either the flagellate Isochrysis galbana (clone T-Iso) or another P. multiseries clone of contrasting cell size. The CR of mussels fed unialgal CLN-50 was also quantified. Particulate DA levels (pDA) ranged from 0.6 to 7.4 pg.cell-1 among clones and trials, but only clones of similar toxicity were combined within mixed diets. When fed the smaller CLN-20 clone (at 5.9 ng pDA.ml-1 of suspension), mussels accumulated weight-specific DA levels ~2.5x higher than oysters (mean peak toxicity = 43.5 and 17.2 Âµg.g-1, respectively). When fed the more toxic but larger CLN-50 clone (at 8.7 ng pDA.ml-1), maximum toxicity remained low in oysters (11 Âµg.g-1), whereas it increased to 243 Âµg.g-1 in mussels (~22x higher than in oysters). Mussels also had a higher CR than oysters (33.1 and 13.7 ml.min-1.g DW-1, respectively). In short-term feeding experiments using mixed diets, oysters showed high ingestion selectivity. They preferentially rejected two larger P. multiseries clones (80-100 Âµm) in pseudofeces when combined with the smaller clone CLN-20 (24-29 Âµm). They also selectively rejected P. multiseries cells when offered in a mixed suspension with I. galbana (4.5 Âµm), regardless of cell size, such that even the small CLN-20 clone was preferentially rejected in pseudofeces. Therefore, low toxicity in oysters fed toxic P. multiseries at bloom concentrations is at least partly explained by a combination of low clearance rate and pre-ingestive rejection of P. multiseries cells at the gills and/or labial palps. The mechanisms for particle selection, which involve differences in size and physical-chemical characteristics of the cells, are the subject of further investigation.