In order to study the influence of fluorine substitution at the meso-position of benz [a] anthracene and at the 12-position of 7-methylbenz [a] anthracene on the carcinogenic properties of these hydrocarbons, the title compounds have been prepared by photocyclization of 1-(1-f1uoro-2-naphthyl)-2-(o-iodophenyl) ethylene and 1-(4-fluoro-1- methyl-2-naphthyl)-2-(o-iodophenyl) ethylene, respectively.