ABSTRACT Rhodococcus sp. strain BCP1, known for its capacity to grow on short-chain n -alkanes (C 2 to C 7 ) and to cometabolize chlorinated solvents, was found to also utilize medium- and long-chain n -alkanes (C 12 to C 24 ) as energy and carbon sources. To examine this feature in detail, a chromosomal region which includes the alkB gene cluster encoding a non-heme di-iron monooxygenase ( alkB ), two rubredoxins, and one rubredoxin reductase was cloned from the BCP1 genome. Furthermore, the activity of the alkB gene promoter (P alkB ) was examined in the presence of gaseous, liquid, and solid n -alkanes along with intermediates of the putative n -alkane degradation pathway. A recombinant plasmid, pTP alkB LacZ, was constructed by inserting the lacZ gene downstream of P alkB , and it was used to transform Rhodococcus sp. strain BCP1. Measurements of β-galactosidase activity showed that P alkB is induced by C 6 to C 22 n -alkanes. Conversely, C 2 to C 5 and >C 22 n -alkanes and alkenes, such as hexene, were not inducers of alkB expression. The effects on P alkB expression induced by alternative carbon sources along with putative products of n -hexane metabolism were also evaluated. This report highlights the great versatility of Rhodococcus sp. strain BCP1 and defines for the first time the alkB gene transcriptional start site and the alkB promoter-inducing capacities for substrates different from n -alkanes in a Rhodococcus strain.