A laboratory study ~n v e s t~g a t e d cell ingestion, absorption of organlc matter, and paralytic shellfish poisoning (PSP) toxin ~ncorporation by iclytilus eduljs exposed to a high-tox~city isolate of the red tide dinoflagellate Alexandrium fundyense (strain GtCA29, toxicity = 66 pg saxltoxin equivalents (STXeq) cell-'). Maximum ingestion rate was achieved at 150 to 250 cells ml-' Clearance rates on A. fundyense were about 48'1.;) lower than those for a non-toxic control diet of the diatom Thalassioslra weissflogii. Mussels with no prior h~story of exposure to PSP maintained a constant ingest~on rate over 17 d of exposure to a A. fundyense (at 256 cells ml-l), and absorbed ca 6 2 % of the organic matter ingested. They experienced no mortality or sublethal adverse effects during intoxication. Maximum (saturation) toxin levels of 4.5 X 10"ig STXeq 100g-' were attained after 12 to 13 d. a value comparable to maximum toxicities reported during major toxic bloom events. Mussels could exceed the quarantine toxin level (80 pg STXeq 100g-l) in < 1 h of exposure to high densities of this isolate. At saturation, they incorporated 79 O/o of the toxin ingested, primarily in the viscera. This provides the first estimate of toxin incorporation efficiency in a bivalve under steady state conditions. Dinoflagellate toxins, determined by HPLC, were dominated by carbamate derivatives. The muscle, mantle/gill and foot of M. edulis showed significant enrichment in STX and reduction in the gonyautoxins GTX2+3 and neoSTX relative to ingested cells. The toxin composition of the viscera more closely resembled that of ingested cells, reflecting the presence of numerous intact cells in gut contents. Through its potential use of A. fundyense as a sole food source, M eduljs is thus capable of remarkably efficient toxln accumulation at environmentally realistic dinoflagellate cell densities.