Various aspects of the energy metabolism of an estuarine mysid (Mysidopsis bahia) were examined for different life stages during a life-cycle exposure to the organophosphate pesticide, fenthion. Dose-response relationships were developed for several metabolic rate functions (oxygen consumption and ammonia excretion) and their combined bioenergetic and physiological indices (K2 values and O:N ratios). Initial exposure to fenthion resulted in elevated respiration rates of juvenile mysids. As shown by lower net growth efficiency (K2 values), these increased metabolic demands reduced the amount of assimilated energy available for production of new tissue, resulting in retarded juvenile growth rates. Results, when compared with similar studies with two other pesticide classes, suggest that measurements of alterations in the energy metabolism of contaminated individuals from sensitive zooplankton populations (e.g. mysids) may be used as indicators of reductions in population performance from chronic exposure to toxic organics.