Abstract: A sensitive bioassay, consisting of two assay techniques applied to embryos of a teleost (bony fish), is utilized in testing for genotoxic agents. The assay allows genotoxic effects to be scored at three different embryo life-stages: (1) before mid-blastula transition (MBT), (2) after MBT, and (3) after mid-gastrula transition. Each embryo life-stage displays different ranges of sensitivity to genotoxic agents. In the first assay technique, embryos are stained for chromosome morphology and then examined, in the form of "flattened whole mounts", by epifluorescence microscopy, for cytogenetic defects. In the second assay technique, embryos are stained for fragmentation of DNA that is associated with apoptosis, and then a flattened whole mount of the embryo is examined. If the result of the cytogenetic assay is negative (with few or none of the cells in the embryo showing cytogenetic defects), then the apoptosis assay can be performed.