Field and laboratory experiments were carried out to investigate the effects of Alberta crude oil and an oil dispersant (Corexit 9527) on the larval settlement, survival, siphon activities and behaviour of the littleneck clam (Protothaca staminea). Corexit 9527 was much more toxic than crude oil, and the highest toxicity was obtained when Corexit 9527 was mixed with crude oil. Siphon activities were impaired and abnormal behaviour was exhibited when adult clams were treated with 100 ppm Corexit 9527, 1000 ppm crude oil or a combination of both. Larval settlement was not affected when the substratum was treated with 1000ppm crude oil but was retarded when the substratum was treated with a mixture of 1000 ppm oil and 100 ppm Corexit 9527. Gas chromatograms also showed that the retention time and depth of penetration of hydrocarbons in the substratum was increased when Corexit 9527 was used with crude oil. Copyright © 1982 Published by Elsevier Ltd.