Bioassays with the marine amphipod Corophium volutator were performed simultaneously in situ and in the laboratory using sediments sampled from the in situ locations. In most cases, the in situ response was significantly higher compared to the laboratory response. This difference was not caused by direct influence of the use of the field chamber on Corophium sp., nor was the difference caused by the overlying water used. Experiments showed homogenization can affect the toxicity of a sediment, but not in such a way that it can completely explain the difference between the response in situ and in the laboratory. Possible explanatory factors are harbor activity, storms, and temperature. To reduce the influence of some of these factors, the best period of the year to perform in situ bioassays with C. volutator is May, June, or September.