Bdellovibrio bacteriovorus is a genus of Gram-negative bacteria that preys on other bacteria. These bacteria have an unusual life cycle consisting of two stages: a motile attack phase and an intraperiplasmic growth phase. Little is known of the genetic and biochemical mechanisms responsible for this developmental "switch." The intracellular regulator cAMP and its receptor protein CRP (catabolite regulatory protein) may play a role in such a genetic switch. Both the cya (adenylate cyclase) and crp (CRP) genes have previously been isolated from our Bdellovibrio genomic library by functional complementation of relevant E. coli mutants. In analyzing phenotypes of Bdellovibrio cosmids in E. coli mutants, it was found that the cosmid carrying the putative Bdellovibrio crp gene appeared to also carry another cya gene. Restriction enzyme and gel electrophoresis analysis of the cosmid containing the cya gene (pMJW100) and the crp gene (pMJW101) showed no overlapping segments, arguing for a possible second cya gene in Bdellovibrio . Yet cAMP assays (using ELISA analysis) demonstrated that pMJW101 did not in fact carry a functional cya gene. This argued that the crp gene of Bdellovibrio is able to activate carbohydrate catabolic gene promoters in the absence of cAMP, which does not occur in E.coli . As Bdellovibrio does not use exogenous carbohydrates for growth or energy in either of its developmental stages, these results are consistent. Further analysis of the cya and crp genes of Bdellovibrio may shed light on gene regulation in this unusual predatory bacterium.