The induction of DNA synthesis in various tissues of Anopheles albimanus, in response to challenge with Saccharomyces cerevisiae, Micrococcus luteus, and Serratia marcescens, was analyzed by 5‐bromo‐2‐deoxy‐uridine (BrdU) incorporation. Microorganism‐inoculated mosquitoes were fed with a sucrose solution containing BrdU and maintained alive for 5 days. Alternatively, abdominal carcasses of microorganisms‐inoculated mosquitoes were cultivated in Roswell Park Memorial Institute (RPMI) medium supplemented with BrdU for 5 days. Control groups were inoculated with RPMI alone. In both experiments, DNA synthesis, evidenced by epifluorescence with an anti‐BrdU fluorescein‐labeled antibody, occurred in fat body, epithelial cells of pleural membranes, dorsal vessel, and the oviducts. Relative quantification of DNA synthesis, evaluated by ELISA using an anti‐BrdU peroxidase‐labeled antibody, was higher in abdomen tissues of microorganisms‐inoculated mosquitoes than controls in in vitro and in vivo experiments. The intensity of DNA synthesis varied among the different microorganism challenges, but was higher in in vivo experiments, compared to cultured samples. These differences in DNA synthesis suggest a compartmentalization of the immune response, probably mediated by different signaling pathways.