Experiments were conducted to assess the effects of copper on larval development in the barnacle Balanus amphitrite. In the first experiment, we compared the sensitivity of three naupliar stages to copper stress. Molting inhibition occurred at copper concentrations ranging from 32 microg l(-1) in nauplius II to 128 g l(-1) in nauplius VI. EC50 for molting ranged from 97 microg l(-1) in nauplius II to 129 g l(-1) in nauplius VI. Decreased survival occurred at 128 microg Cu l(-1) in all of the naupliar stages tested, with LC50 ranging from 145 in nauplius II to 213 microg l(-1) in nauplius VI. In the second experiment, we examined effects of copper on the development from nauplius II to cyprid. The larvae reached cypris stage only in treatments of 16 microg Cu l(-1). Our study therefore showed that molting was a more sensitive endpoint than survival, nauplius II was the most sensitive naupliar stage, and that whole larval development assay was more sensitive than assays using a particular larval stage. The results were discussed with respect to the use of this species in toxicity tests.