Filter-feeding behaviours of the mussel Perna viridis and the oysters Crassostrea belchen, Crassostl-ea ~radelei, Saccostrea cucculata and Pinctada mal-garifera were compared during natural tidal vanations in the concentration (6 to 40 dry mg total particulate mass I-') and organic content (6 to 2 2 % ) of seston available in the Merbok mangrove system, Malaysia. In P. viridis, C. belcheriand S. cucculata, declining retention eff~ciencles for the largest available particle slze classes of more than 6 to 12 pm diameter were associated w t h an overall organic enrichment of filtered relative to available matter. As seston avdilab~lity increased, a ~n i n~m u m average of 71°,. of the additional filtered matter was rejected by each species as pseudofaeces prior to ingestion. And in all 5 species, preferential rejection as pseudofaeces of filtered particles with higher average inorganic content resulted in the net organic enrichment of ingested relative to filtered matter. In P. viridis and C. belcheri, the efficiency of that net organic selection declined for seston of lower organlc content. Combining all data from each species, absorption efficiencies from ingested organics increased exponent~ally with the organic content of ingested matter Collective findings suggest that key interrelations between component processes of nutrient acquisition were similar to those that have been established for the modeling of feeding, growth and environmental ~mpact among filter-feeding bivalves froin temperate latitudes However, especially fasl growth in P viridis stemmed from higher average (* 2 SE) clearance rates (13.2 A 7.0 l h-' g-' when particulate organic matter c 1 mg I-'), greater average organic enrichment of filtered relative to available matter (0.99 + 0.67) and greater average organic enrichment of ingested relative to filtered matter (0.63 * 0.03) than have hitherto been recorded in any species of bivalve filterfeeder. In contrast, P. margarifera was least well adapted for the pre-ingestive selection of organic matter, with no differential retention on the gill and little selective rejection as pseudofaeces, representing at least part of the physiological basis for characteristically slow growth in pearl oysters.