Life-cycle and embryo-larval toxicity tests were conducted on the fathead minnow (Pimephales promelas) with three commercial linear alkylbenzene sulfonates (LAS), anionic surfactants commonly used in laundry detergents. The no-observed-effect concentration (NOEC), as measured in life-cycle tests or estimated from embryo-larval tests, depended on the mean alkyl chain length of the LAS and was 5.1–8.4, 0.48, and 0.11–0.25 mg/liter for C11.2, C11.7, and C13.3LAS, respectively. The embryo and larval stages of development were consistently the most susceptible. The concentration of methylene blue active substances (MBAS), a nonspecific measure of anionic surfactants, was determined in over 800 widely distributed United States stream samples. The mean alkyl chain length of LAS in municipal sewage treatment effluents was C11.5. On the assumption that LAS contributes approximately 50% of the stream MBAS, 96 to 100% of the stream samples contained LAS at concentrations below the measured or estimated NOEC for fathead minnows.