Measures of physiological performance and energy metabolism were made on an estuarine mysid (Mysidopsis bahia) exposed throughout a life cycle to the defoliant DEF. DEF concentrations > or = 0.246 microgram/l reduced survival through release of the first brood. Young production was completely arrested in DEF concentrations > or = 0.606 microgram/l, while concentrations > or = 0.085 microgram DEF/l reduced young production by the population. Concentrations > or = 0.140 microgram/l and > or = 0.085 microgram/l retarded growth rates in older juveniles and young adults, respectively, while stimulating respiration rates. Young mysids excreted less ammonia in DEF concentrations > or = 0.085 microgram/l. Higher O:N ratios during maturation of DEF-exposed mysids suggests greater reliance on energy-rich lipid substrates to support elevated metabolic demands, leaving reduced lipid for reproductive preparations.