The study used the Photobacterium phosphoreum bacterial luminescence assay (Microtox R) to survey the distribution of the toxicity of the sediments from the lower Detroit River during the summer of 1986. Of the 136 locations tested, 25 were classified as very toxic, 60 as moderately toxic, 10 as slightly toxic, and 41 were classified as being non-toxic. The greatest number of very toxic sites was observed on the western shore of the Trenton Channel; however, some very toxic locations were observed throughout the study area. The least toxic areas were observed at the eastern-most locations studied. The Microtox R assay has been found to be a sensitive assay, which can be calibrated to the responses of other organisms such as macroinvertebrates and can be related to the potential for distribution of macroinvertebrates in sediments. Therefore, the Microtox R assay is useful for rapid screening and mapping of toxicity of sediments.