AbstractThe epibenthic mysidMysidopsis bahiawas chronically exposed throughout its entire life cycle to inorganic mercury. The experimental design permitted the precise measurements of survival rates; individual and population growth rates; and sublethal reproductive responses, including time to sexual maturation and first brood release, egg development time, brood size and frequency of reproduction. Mercury was acutely toxic (96‐h LC50) to juveniles at 3.5 μg L−1, chronically toxic (35‐d LC50) at 1.8 μg L−1and differentially toxic to males and females (α = 0.05). Male and female mortality rates were 35 and 18%, respectively, from sexual maturation (day 14) through termination of the assay (day 35). Reproductive effects included delays in sexual maturation and brood release at 1.6 μg L−1, a doubling of brood development time at 2.5 μg L−1and subsequent abortion and significant decreases in the total broods released (and therefore juveniles produced) at 1.6 μg L−1. Intrinsic rates of population increase (r), calculated from life‐tables, decreased with increasing mercury concentration. The critical value (r = 0) for mercury occurs at 1.6 μg L−1, which corresponds well with the upper limit of the maximum acceptable toxicant concentration, the usual endpoint of life‐cycle toxicity tests. The toxicity test system is a sensitive, precise model for studying lethal, sublethal and potential population