Test compounds including natural hormones, endocrine disrupters, environmentally occurring compounds, and reference compounds were tested for acute toxicity and inhibitory effect on larval development in the copepod Acartia tonsa. Three compounds, 17alpha-ethinylestradiol, p-octylphenol, and tamoxifen, known for their differing effects on the vertebrate estrogen system, were potent inhibitors of naupliar development. Other estrogens, 17beta-estradiol, estrone, and bisphenol A, had little potency. Testosterone and progesterone did not inhibit development, but the antiandrogen flutamide had inhibitory effect. Juvenile hormone III was a potent inhibitor, as was expected based on the literature, whereas 20-hydroxyecdysone had no effect. 3,4-Dichloroaniline was inhibitory on development, whereas other control compounds, potassium dichromate and 3,5-dichlorophenol, did not inhibit development. Six of the 17 test compounds had 50% lethal concentration to 50% effective concentration (EC50) ratios higher than 10. The results suggest that naupliar development, as a parameter, is able to detect hormonal disrupters in addition to other chemicals that have other specific modes of action.