Six pesticides were evaluated in laboratory studies to determine acute (96-h) toxicity, octanol-water partition coefficient (log P), solubility, and persistence in seawater. In addition, three of the six pesticides (synthetic pyrethroids) were tested by using the eastern oyster (Crassostrea virginica) in long-term (28-day) tests to determine their respective bioconcentration factors (BCF). Acute toxicity tests provided the following decreasing order of toxicity to estuarine crustaceans and fishes: AC 222,705, fenvalerate, permethrin, chlorpyrifos, methyl parathion, and benthiocarb. The estuarine mysid (Mysidopsis bahia) was consistently the most sensitive species, with LC50 values as low as 0.008 micrograms/L. The sheepshead minnow (Cyprinodon variegatus) was generally the least sensitive (range of LC50 values = 1.1-1370 micrograms/L). Log P values were inversely related to solubility in seawater.