The exposure of non-target aquatic organisms to sublethal levels of combinations of environmental pollutants may result in synergistic, antagonistic or additive effects. Short-term exposures of planaria to sublethal concentrations of malathion, carbaryl and 2,4-D alone, and in combination, resulted in alterations in both neurobehavior and acetylcholinesterase (AChE) levels. Both interactive and individual effects of the pesticides were concentration dependent. Pesticide mixtures produced varied interactive effects. At very low levels, all pesticide mixtures produced significant (p < 0.01) synergistic responses in the neurobehavior of planaria and resulted in a greater number of responses than predicted additive values. Responses obtained from exposure to low levels of the pesticide mixtures showed a less than additive increase. Malathion and carbaryl exposure resulted in 5–45% decreases in AChE activity. AChE levels increased 6–34% after 2,4-D exposure. Evaluation of planarian neurobehavior responses provided a simple, sensitive method for biomonitoring the interactive sublethal toxicity associated with pesticide mixtures.