This study was part of a characterization of the nature and severity of water-quality problems in the Hudson/Raritan Estuary in New York State and New Jersey, USA. The toxicity of ambient water was measured at 51 stations in the estuary by using standard tests with the sea urchin Arbacia punctulata and the marine red alga Champia parvula. Toxicity identification evaluations on samples from two stations suggested that cationic metals were the source of the observed toxicity. Overall results showed that toxicity could vary as much on the small scale, i.e., with depth and tide at a single site, as over several stations within a given subarea of the estuary. Thus, knowing about small-scale variations in toxicity is essential to understanding the significance of the variations from different areas or different sampling events.