Results are reported for laboratory experiments using the mysid shrimp, Mysidopsis bahia, as a practical test species for life-cycle studies to determine subtle effects of toxicants. Ratios of the acute 96-hour lethal concentrations to the maximum acceptable toxicant concentration (MATC) ranged from 0.01 to 0.54, indicating that, in some instances, acute tests would be predictive of chronic effects. Criteria used to establish MATC limits were effects on growth, reduced reproduction, chronic toxicity and, with two chemicals, reproduction and chronic toxicity, concurrently. Though the data base is incomplete, the acute tests with M. bahia and 11 pesticides show sensitivities within a factor of 10 to some decapod crustaceans--most of them being penaeid shrimp. Acute studies conducted elsewhere show M. bahia to be as sensitive to, or in many instances, more sensitive than five fresh water and marine species commonly used in toxicity testing.