A polycyclic aromatic hydrocarbon recently isolated from carbon black and identified as cyclopenta[c,d]pyrene (CPP) is highly mutagenic. By the criteria of the Salmonella/mammalian-microsome mutagenicity test, the mutagenic potency of CPP is equalled by only two other naturally occurring polycyclic aromatic hydrocarbons--benzo[a]pyrene and dibenz[a,c]anthracene. The potent mutagenicity of CPP is noteworthy for two reasons: (i) CPP is a mutagenic polycyclic aromatic hydrocarbon without a "bay-region" and (ii) there is evidence that it is distributed widely in the environment. On the basis of experimental observations and perturbational molecular orbital calculations we propose that a mutagenic metabolite of CPP will be the 3,4-oxide. The carbonium ion derived from opening of CPP 3,4-oxide is identical to that derived from opening of benzo[a]pyrene 7,8-diol-9,10-oxide, the metabolite now thought to be an ultimate mutagenic and carcinogenic species.