Early life-stage toxicity tests (continuous exposure from embryonic stage to approximately three weeks or more into the exogenous feeding stage) with North American marine fishes have been conducted almost exclusively with cyprinodontids. In this paper, the authors present methods for testing a representative of an additional family, Atherinidae. Embryos of the tidewater silverside, Menidia peninsulae, were obtained by a laboratory spawning procedure that required lighting and tidal (current) cues. The 28-day toxicity test with chlorine produced oxidants (CPO) began with stage 21 and 22 embryos (approximately 36-h old). Average measured CPO concentrations in exposure water were: nondetectable (<0.01 mg/L) in the control and the two lowest exposure concentrations; and 0.01, 0.04, and 0.21 mg/L. Survival of embryos to hatching averaged 99%, with no significant difference among treatments.