Several polycyclic hydrocarbons, 3,6-dimethoxy-9,10-bis(p-methoxyphenyl)-phenathrene, tetrakis(p-methoxyphenylyethylene and 3,6,11,14-tetramethoxydibenzo(g,p)chrysene, which are associated as impurities in commerical samples of the insecticide methoxychlor, have been tested in the Ames mutagenicity test with strains of Salmonella thyphimurium, TA 1535, TA 1537, TA 1538, and TA 98. Activation by liver microsomes induced with either phenobarbitol or Aroclor was examined. The only active compound was 3,6,11,14-tetramethoxydibenzo(g,p)chrysene, mutagenic (0.39 revertants/nmol) tostrain TA 98.