The toxicity of nine pesticides to various life history stages of the Dungeness crab, Cancer magister, was examined to establish the most sensitive life stage of the crab, and the highest concentration of each pesticide having no discernible effect on that most sensitive stage during prolonged exposures. The compounds tested were the insecticides carbofuran, chlordane, malathion and methoxychlor; the herbicides 2,4-D, DEF, propanil and trifluralin; and the fungicide captan. For each pesticide the zoeal stages were found to be the most sensitive in long-term tests, approximately 5 to 10 times and 10 to 100 times more sensitive than juvenile and adult crabs, respectively, and were also affected at lower concentrations than those that affected egg hatching and prezoeal development. The maximum acceptable toxicant concentrations for continuous exposures of C. magister zoeae to each of the nine pesticides are: methoxychlor, 0.005 microgram/liter; chlordane, 0.015 microgram/liter; malathion, 0.02 microgram/liter; carbofuran, 0.05 microgram/liter; captan, 2 micrograms/liter; DEF, 4 micrograms/liter; trifluralin, 15 micrograms/liter; propanil, 80 micrograms/liter; and 2,4-D 1000 micrograms/liter. The toxicity of each of these pesticides to crabs is compared with literature reports of their toxicity to other aquatic species.