Chronic bioassays were used to evaluate the concentration and exposure duration of methylmercury that resulted in specific teratogenic defects in Danio rerio embryos exposed at different developmental stages. Embryos in different stages of development (cleavage, blastula, gastrula, or segmentation) were exposed to 20 or 30 µg/l of methylmercuric chloride (CH(3)HgCl) for various exposure durations (8, 16, 32 h, or continuously to hatching). These exposures frequently caused two morphological defects, tissue abnormality in the median finfold and a flexure of the posterior tail region. The critical period of exposure for the production of both effects begins around 18-20 h after fertilization, with increased exposure resulting in more severe effects. These critical periods coincide with both tail and median finfold formation.