The influence of food quantity on the effects of chlorpyrifos was determined in six early life-stage (ELS) toxicity tests with estuarine sheepshead minnows, Cyprinodon variegatus. Three ELS tests were conducted simultaneously, each with a different feeding density: approximately 20, 110 or 550 Artemia nauplii/fish per feeding. In the first series of three tests, growth was significantly reduced (p greater than or equal to 0.001) at nearly all feeding densities and concentrations tested (3.1 to 52 micrograms chlorpyrifos/l). Therefore, a second series of tests was conducted at lower chlorpyrifos concentrations that ranged from 0.4 to 6.8 micrograms/l and at the same feeding densities. At test termination bioconcentration factors (amount of chlorpyrifos in tissue divided by average measured water concentrations) and chlorpyrifos in whole fish at exposure concentrations greater than or equal to 3.0 micrograms/l generally increased with increasing Artemia densities and increasing chlorpyrifos concentrations.