Abstract-The distribution and sources of haloacetic acids in seawater in the vicinity of Tokyo Bay, Japan, were investigated, in view of the potential toxicity of these compounds to aquatic organisms. Haloacetic acids were extracted from water samples collected at 66 sampling stations (n ϭ 122). Concentrations of all nine haloacetic acids containing bromine and chlorine were determined by liquid chromatography-mass spectrometry. We found that concentrations of trichloroacetic acid (TCAA) were high (maximum: 20.3 g/L) compared to those of the other haloacetic acids in coastal seawater. The distributions and relative concentrations of TCAA and other haloacetic acids (monochloroacetic acid, dichloroacetic acid, and bromochloroacetic acid) suggest that their sources are not identical; discharge of wastewaters containing TCAA in the bay area is probably one of the main sources of the observed high concentrations of TCAA in seawater there. The mean concentrations of haloacetic acids in water samples and the results of incubation experiments of haloacetic acids in river water and seawater suggest that TCAA is relatively stable in the aquatic environment.