Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Male-killing bacteria are maternally inherited agents that cause death of sons of infected females. Their transmission rate is commonly high but imperfect and also sensitive to different environmental factors. Therefore, the proportion of infected females should be reduced in each generation. In order to explain male-killers spread and persistence in host population, a mechanism resulting in the relative increase of infected females must outweigh the losses caused by the imperfect transmission. The resource release hypothesis states that the males' death results in increased resources available to sibling females which would otherwise be used by their male siblings. Infected females are then expected: to be larger than uninfected females in natural populations; or to have higher viability; or to have shorter development times; or any combination of these outcomes. Here, we tested the resource release hypothesis by measuring body size of infected and uninfected wild-caught Drosophila melanogaster females and carried out other fitness related measures in the laboratory. Wild-caught infected females produced more daughters than uninfected females in their first days in the laboratory. However, although no significant difference in viability was found in a controlled experiment with infected and uninfected flies from a standard laboratory strain, there was a decrease in development time probably mediated by reduced competition. Fitness effects conditioned by the host genetic background are pointed out as a possible explanation for this difference between wild and laboratory flies. Our findings are discussed in the context of the resource advantage hypothesis. (C) 2010 Elsevier Inc. All rights reserved.1052145150Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundo de Apoio ao Ensino, a Pesquisa e a Extensao - Unicamp (FAEPEX - Unicamp)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)