Four groups of 15 male and 156 female Sprague-Dawley rats were exposed, six hours per day for five consecutive days, to either clean air or 153, 471 or 1,540 ppm of C(sub 9) aromatic hydrocarbon vapor. A fifth group of five males and five females served as positive controls and received an injection of 40 mg/kg cyclophosphamide. Five males and five females from each exposure group were injected with 4 mg/kg colchicine at 6, 24 and 48 hours after the last exposure. Six hours after the colchicine injection, the animals were sacrificed, bone marrow collected, and chromosome spreads prepared. The positive control animals were evaluated at the 24-hour interval. Under the exposure conditions used for this study, C(sub 9) aromatic hydrocarbon vapor did not induce chromosome or chromatid abberations at any exposure level, and therefore, was considered to be non-mutagenic.