AbstractShort‐chain, 2‐ and 3‐carbon halogenated hydrocarbons were tested for mutagenicity for Salmonella typhimurium strain TA 100 both with and without the presence of S‐9. Without exception, all brominated derivatives were more mutagenic than the chlorinated derivatives, usually by a substantial order of magnitude. 2‐Fluoroethanol, the only fluorinated compound tested, showed little or no mutagenic activity up to 100 μmole per plate concentration. Two highly purified propane derivatives containing a halogen atom on each of the three carbons showed little or no direct mutagenic activity. A third trihalogenated compound with a halogen atom on each carbon atom showed some direct mutagenic activity, probably due to impurities. However, all three trihalogenated compounds were highly active mutagens following S‐9 activation. The presence of a double bond in the case of 1,2,3‐trichloropropene resulted in a higher level of direct mutagenic activity than 1,2,3‐trichloropropane, but activation with S‐9 resulted in a further increase in mutagenic activity with the former compound. On the other hand, S‐9 caused a substantial decrease in mutagenic activity of most compounds containing a double bond. With the presence of an alcoholic group in a compound, the addition of S‐9 caused variable responses, increasing the number of his+revertant colonies due to 2,3‐dibromopropanol but had little or no effect with five other compounds containing an alcoholic group. Evidence is also presented that the position of a double bond in relation to the halogen atoms may infl