\'In contrast to previous findings, we showed that gene deletion Pfcrk5- parasites grew normally as asexual stages and underwent normal gametocytogenesis to stage V gametocytes. However, Pfcrk5- parasites showed a severe defect in male gametogenesis, which was evident by a significant reduction in the emergence of male gametes (exflagellation). This defect caused a severe reduction of parasite transmission to the mosquito. Genetic crosses performed using sex-specific sterile transgenic parasites revealed that Pfcrk5- parasites suffered a defect in male fertility but female gametes were fertile. Taken together, these results demonstrate that PfCRK5 is a critical sexual stage kinase which regulates male gametogenesis and transmission to the mosquito.\'