1. Microorganisms that manipulate the reproduction of their hosts through diverse mechanisms including the induction of parthenogenesis are widespread among arthropods. 2. The pea aphid, Acyrthosiphon pisum, shows a variation in its reproductive mode, with lineages reproducing by cyclical parthenogenesis (obligate alternation of parthenogenetic and sexual generations each year) and others by obligate parthenogenesis (continuous asexual reproduction all year round). In addition, the pea aphid harbours, along with Buchnera the primary aphid endosymbiont, several facultative symbionts whose prevalence differs among host populations. 3. The possible influence of a Rickettsia facultative symbiont on the reproductive mode of its host was tested on two pea aphid clones by comparing the response of infected and uninfected individuals with the same genetic background to conditions that typically induce the production of sexual morphs. 4. No significant effect of the Rickettsia infection was found on the type of reproductive morphs produced (sexual vs. asexual) or on their quantities for the two clones. 5. However, the Rickettsia had a detrimental effect on the fitness of its aphid host, in apparent contradiction to the high prevalence of this symbiont in some host populations. It is suggested that this negative impact may disappear under specific environmental conditions, transforming a parasitic association into a mutualistic one