Cyclopenta-fused polycyclic aromatic hydrocarbons (PAH) are a unique class of PAH found in the environment. Acenaphthylene, acephenanthrylene and cyclopenta (cd) pyrene represent characterized cyclopenta-PAH already identified as air pollutants. The pyrolytic synthesis of PAH from two carbon fragments (3) suggests that many more such cyclopenta-ring fusions are possible and may be characterized from environmental samples. Cyclopenta-PAH are non-alternate PAH in which the fused five membered ring provides a new site for metabolic attack by the cytochrome P-450 mixed-function oxidases. The study of the metabolism, metabolic activation and mutagenesis of these chemicals allows a probe into the mechanism of oxygen transfer and the stereo-and regio-specificity of the cytochrome P-450 mixed-function oxidases as well as an understanding of the competition between sites of metabolic action by these enzymes. This chapter is a preliminary report of metabolism and mutagenesis studies with four cyclopenta-fused isomers of benz(a)anthracene: benz(j)aceanthrylene, BjA; benz(e)aceanthrylene, BeA; benz(1)aceanthrylene, B1A; and benz(k)acephenanthrylene, BkA.