The report presents the following conclusions: (1) Exposure of the estuarine shrimp, Palaemonetes pugio, to a juvenile hormone analogue (> or = 8 microgram methoprene/l) throughout larval development inhibited successful completion of metamorphosis; (2) Methoprene exposure retarded growth in early larval stages and postlarvae, but enhanced growth in premetamorphic larvae; (3) Respiration rates of early larvae were elevated by methoprene exposure, but not so older larvae or postlarvae; (4) Lower net growth efficiency (K2 values) in methoprene-exposed early larvae suggests that increased metabolic demands reduced assimilated energy available for growth; and (5) Modifications in O:N ratios of premetamorphic larvae and postlarvae suggest that methoprene altered substrate utilization patterns during metamorphosis.