A benthic assay was developed utilizing time-lapse photography to measure the feeding activity of a lugworm, Arenicola cristata. Automated 35 mm cameras were used to record formation of feeding funnels at 12-hour intervals. Substrate surface area reworked by lugworms held under identical conditions in separate aquaria was plotted against time to determine substrate reworking rates for each group. Rates were subjected to linear regression analysis and compared to demonstrate that no significant difference between the slopes of the calculated lines existed. Therefore, a difference in slope when one group is exposed to a toxicant could provide a measure of effect on lugworm activity. Lugworms also were exposed to the pesticide, Kepone, and their rate of substrate reworking was compared with unexposed lugworms. Kepone was acutely toxic to lugworms at a concentration of 29.5 micrograms/l. A significant difference in substrate reworking rates was observed following exposure to concentrations as low as 2.8 micrograms/l Kepone in seawater. It is suggested that a behavioral response to toxicity testing provides a sensitive and realistic approach for evaluation of ecological impact of pollutants on the marine environment.