The mutagenicity of aliphatic (fraction J), one‐ to three‐ring aromatic (fraction 3), and four‐ to five‐ring aromatic (fraction 4) fractions of Prudhoe Bay crude oil and of a fuel oil no. 2 was investigated with a battery of in vitro mammalian and microbial assays. Chemical analysis of the fractions was carried out by gas capillary chromatog‐raphy‐mass spectrometry. In the Salmonella/mammfl//a/7 microsome test, both first fractions were negative, while fraction 3 of Prudhoe Bay crude and fraction 4 of both oils exhibited mutagenic activity with metabolic activation. In Chinese hamster ovary cells, all of the aromatic fractions, in the presence of metabolic activation, caused a significant increase in the number of sister chromatid exchanges. Chromosome aberrations were not caused by any of the aromatic fractions; however, fraction 3 of fuel oil no. 2 induced endoreduplication in the presence of metabolic activation.