Methoxychlor (MXC) is a widely used pesticide which has been found in water sources near agricultural sites. Embryos of aquatic organisms are likely to encounter MXC due to land runoff. The sea urchin embryo (Strongylocentrotus purpuratus) was used as a model system to document the effects of MXC on early development up to the pluteus stage. Fertilized eggs and embryos were exposed to several concentrations (0.1, 1.0, 10, and 100 ppm) of the pesticide in both chronic and acute exposure regimens. With chronic exposure, percentages of embryos completing normal first cleavage decreased with increased concentrations of MXC, and subsequent cleavages became even more irregular in that blastomeres divided asymmetrically and asynchronously. Ten parts per million MXC allowed development through the hatched blastula stage, whereas embryos in 100 ppm MXC did not hatch. In acute exposure trials, fertilized eggs were pulsed (i.e., exposed for brief durations) to MXC for 30, 60, or 90 min. The MXC was then washed out. Recovery of normal development was proportional to the amount and duration of MXC exposure. Development was delayed in embryos exposed to 100 ppm MXC for 30 or 60 min. The embryos exposed to 100 ppm MXC for 90 min were abnormal as early as the four-cell stage, and by 72 hr more than 90% had abnormal gut development, indicating disruption of gastrulation. These data show that MXC exposure resulted in retardation of cleavage and abnormal gastrulation, basic morphogenetic processes.