Much attention is focused on environmental contamination by heavy metals. The heavy metal mercury is found worldwide and is ranked number 3 on the Comprehensive Environmental Response, Compensation and Liability Act substance list. We examined the effect of low-level methylmercury exposure on central nervous system development of wild-type zebrafish embryos (ZFEs) of the AB strain because methylmercury is the most common form of mercury to which humans are exposed in the environment. ZFEs were exposed to nine different concentrations of methylmercury [0 (negative control), 5, 10, 50, 80, 100, 200, 500 and 1000 parts per billion (μg l(-1) )] starting at 6 h post-fertilization, which is the time the neural tube is first beginning to form. ZFEs were exposed to 2% ethanol as positive controls (100% embryonic death). ZFEs were assessed at 30, 54, 72 and 96 h post-fertilization for changes in embryonic development, mortality, time of hatching and morphological deformities. No abnormalities were observed in ZFEs exposed to 5 μg l(-1) methylmercury. The time of hatching from the chorion was delayed in ZFEs exposed to methylmercury concentrations of 50 μg l(-1) or higher. Significantly more ZFEs exposed to 0, 5 or 10 μg l(-1) methylmercury successfully completed hatching compared with ZFEs exposed to 50 μg l(-1) or higher methylmercury. ZFEs exposed to more than 200 μg l(-1) methylmercury exhibited 100% embryonic mortality. The rate of cell proliferation within the neural tube was significantly decreased in embryos exposed to 10, 50 and 80 μg l(-1) methylmercury and there were no differences between these doses.