The transcriptome of Blochmannia floridanus, the endosymbiont of the carpenter ant Camponotus floridanus, is presented during various developmental stages of its holometabolous host by use of a whole-genome DNA macroarray. The detected transcription patterns indicate the presence of local transcription units as well as global regulatory mechanisms. Yet, the overall regulation scale is very modest, rarely exceeding a factor of three. A large number of genes show differential expression in different life stages and a distinct expression pattern of genes possibly involved in symbiotic function as compared with housekeeping genes is apparent. However, these transcriptional changes are small as compared with the changes in the number of bacteria during host development, which is the highest in pupae and in young imagines. Control of replication of the bacteria in certain life stages may therefore be the decisive parameter influencing the overall level of gene expression of Blochmannia in the animal. The few highly expressed genes like those encoding molecular chaperones exhibit a significantly higher G+C content than moderately expressed genes.