The expression of β‐galactosidase by the lac transposon Tn951, in Escherichia coli, was found to be cAMP‐dependent. This finding provided the basis for an investigation of the effect of cAMP on Tn951 lac expression in Rhizobium, with the ultimate aim of using the Tn951 system as a specific probe for cAMP mediated catabolite repression. When introduced into Rhizobium, Tn951 directed the synthesis of β‐galactosidase, which was inducible by isopropyl‐β‐d‐thiogalactopyranoside (IPTG). Marked quantitative and qualitative differences in β‐galactosidase expression were found between R. meliloti and R. japonicum during the growth cycle, with expression being higher in the former. β‐Galactosidase levels were, however, unaffected by exogenous cAMP under catabolite repressing conditions.