The soil isolate, Mycobacterium sp. strain KMS, utilizes an array of carbon compounds including the aromatics benzoate and pyrene as sole carbon sources. Growth on pyrene induced both chromosomal and plasmid nidA genes encoding pyrene ring-hydroxylating dioxygenase α-subunits for pyrene oxidation. Diauxic growth occurred when KMS was cultured with pyrene plus either acetate, succinate, fructose, or benzoate and nidA expression only was detected in the second slower log-phase period. Potential cAMP-CRP binding sites exist within the promoter region of both nidA genes indicating that cAMP-CRP may be involved in catabolite repression of pyrene utilization. When cultured with benzoate plus either acetate, succinate, or fructose, there was no diauxic growth. Also there was no diauxic growth on fructose plus succinate or acetate. Expression of a benA gene, encoding a benzoate dioxygenase α-subunit involved in the initiation of benzoate oxidation, was detected in log-phase cells from the benzoate-mixed substrate cultures at the same level as when the cells were cultured on benzoate alone. These findings suggested that catabolite repression of pyrene but not benzoate occurred in isolate KMS. These differences may help the microbe exploit the varied carbon sources available in the soil and rhizosphere environments.