Toxicity of three household soaps to mummichogs (Fundulus heteroclitus) was determined at 23% salinity and 20[degree]C. Concentrations which killed 50% (LD50) in 96 hours ranged from 750 ppm for soap I to 1,540 ppm for soap L. LD50 values (96 hours) of L for mullet (Mugil cephalus) and for silversides (Menidia menidia) were 580 and 790 ppm, respectively. Salinity of the medium affects solubility of soaps. Soaps were most soluble at salinities approaching fresh water, but formed precipitates and insoluble complexes with increasing salinity. Mummichogs exposed to a high concentration of each soap experienced heavy mortality at test salinities of 21% and lower. Few or no deaths were observed at higher salinities.