1.1 ,This guide describes procedures for obtaining laboratory data concerning the acute effects of a test material on embryos and the resulting larvae of four species of saltwater bivalve molluscs (Pacific oyster, Crassostrea gigas Thunberg, eastern oyster, Crassostrea virginica Gmelin, quahog or hard clam, Mercenaria mercenaria Linnaeus, and blue mussel, Mytilus edulis Linnaeus) during static 48-h exposures. These procedures will probably be useful for conducting static acute toxicity tests starting with embryos of other bivalve species (1)² although modifications might be necessary.