Three short-term assays (the SOS chromotest, the Ames-fluctuation test and the newt micronucleus test) were carried out to evaluate the genotoxicity of six halogenated acetonitriles likely to be found in chlorinated waters: monochloro-, dichloro-, trichloro-, monobromo-, dibromo- and bromochloroacetonitrile.With the SOS chromotest, three of the chemicals studied (dichloro-, dibromo- and bromochloroacetonitrile) were found to induce primary DNA damage in vitro on Escherichia coli PQ37. In the fluctuation test, all the compounds except dibromoacetonitrile showed mutagenic activity in vitro onSalmonella typhimurium strain TA100. The newt micronucleus assay detected, in vivo, a clastogenic effect on the peripheral blood erythrocytes of Pleurodeles waltl larvae for all the six haloacetonitriles studied. Moreover, this in vivo test was the most sensitive of the three assays for the detection of the genotoxicity of every haloacetonitrile studied.Two structure-activity relationships were noted: (1) the genotoxicity activity of haloacetonitriles containing bromine substituents appeared higher than the corresponding chlorinated acetonitriles, and (2) the clastogenic activity of the chlorinated acetonitriles increased with the number of chlorine substituents.