1. Aromatic amino acid decarboxylase activities toward L-DOPA (L-3,4-dihydroxyphenylalanine), 5-HTP (5-hydroxytryptophan) and p-tyrosine in different tissues of the sclerotized and newly ecdysed cockroach were analyzed. 2. The ratios of enzyme activity with regard to L-DOPA and p-tyrosine varied considerably in the tissues and between the two different growth stages. 3. A DOPA decarboxylase and a p-tyrosine decarboxylase were separated by gel filtration and ion exchange chromatography. 4. The optimal pH requirement for both enzymes was 7.5 with the exception of the one decarboxylating 5-HTP. 5. The molecular weights of the cockroach brain DOPA decarboxylase and tyrosine decarboxylase were estimated to be 120,000 and 100,000, respectively. 6. Unlike the mammalian aromatic amino acid decarboxylase, the cockroach DOPA decarboxylase cannot be activated by a small amount of benzene. 7. An increase of over 50-fold of DOPA decarboxylase activity and a 50% reduction of tyrosine decarboxylase activity in the epidermal tissue of the newly ecdysed animals was observed. 8. In the fully sclerotized cockroach, a reversible endogenous inhibitor(s) of DOPA decarboxylase in the integument was observed, suggesting that the DOPA decarboxylase is suppressed in the epidermal tissues when ecdysis does not occur.