The broad-host range plasmid pAW63 is a model for the study of molecular mechanisms associated with conjugation in the Gram-positive Bacillus cereus group. Its main features are a conjugative apparatus that includes Type IV Secretion System-like components and two Group II introns, B.th.I1 and B.th.I2, located within conjugation genes, as well as a putative regulatory control circuit. Furthermore, pAW63 shares a common backbone with pXO2, the second virulence plasmid of Bacillus anthracis, and with pBT9727 from the pathogenic Bacillus thuringiensis subsp. konkukian strain 97-27. In this study, the transcriptome of pAW63 was investigated using a custom DNA microarray, providing insight into the genetic clockwork of this conjugative plasmid. Gene expression profiles suggested that in the absence of mating partners, a partial 'standby mode' was in effect, with little production of many of the structural elements thought to be involved in mating pair formation and DNA transfer, while components of a proposed quorum sensing mechanism were actively expressed. Intron splicing was demonstrated for the B.th.I2 intron.