Parthenogenesis-inducing Wolbachia bacteria are reproductive parasites that cause infected female wasps to produce daughters without mating. This manipulation of the host's reproduction enhances the transmission of Wolbachia to future generations because the bacteria are passed on vertically only from mothers to daughters. Males are dead ends for cytoplasmically inherited bacteria: they do not pass them on to their offspring. Vertical transmission of Wolbachia has been previously considered to be the main mode of transmission. Here we report frequent horizontal transmission from infected to uninfected wasp larvae sharing a common food source. The transferred Wolbachia are then vertically transmitted to the new host's offspring. This natural and unexpectedly frequent horizontal transfer of parthenogensis-inducing Wolbachia intraspecifically has important implications for the co-evolution of Wolbachia and their host.