The influence of pesticide-sediment associations on pesticide-algae interactions was investigated by examining the inhibitory effects of 3 pesticides on algal photosynthesis in one compartment (axenicEuglena gracilisculture) and 2 compartment (alga plus sediment) systems. Carbon-14 incorporation was used as an indication of photosynthetic activity. A 24-h exposure to either DDT or carbaryl significantly inhibited photosynthesis inE. gracilisat concentrations of 10 ppb or greater. Abate® [O,O'-(thiodi-p-phenylene)O,O,O',O'-tetramethyl phosphorothioate] was only marginally inhibitory at concentrations up to 100 ppb. The presence of sediment was highly significant in reducing the inhibition for all 3 insecticides. This effect of increasing test system complexity is important in extrapolating results of simple laboratory experiments to field conditions.