The different genetic end-points which can be tested to detect genotoxicity of chemicals in fungi and especially in the yeast Saccharomyces cerevisiae are described. They include reversion and forward mutation, mitochondrial deletions and point mutations, mitotic or meiotic intra- and intergenic recombination, chromosomal non-disjunction and aneuploidy. Several factors known to affect the response to genotoxic agents such as the growth parameters, the repair ability, the cells permeability, etc., are discussed. The recent validation studies on the mutagenic and recombinogenic activities of a number of chemicals indicate that within the battery of rapid, low cost and quantitatively reliable tests, the yeast system can be profitably used.