<jats:p>The recently discovered <jats:italic>Anopheles</jats:italic> symbiont, <jats:italic>Microsporidia MB</jats:italic>, has a strong malaria transmission-blocking phenotype in <jats:italic>Anopheles arabiensis</jats:italic>, the predominant <jats:italic>Anopheles gambiae</jats:italic> species complex member in many active transmission areas in eastern Africa. The ability of <jats:italic>Microsporidia MB</jats:italic> to block <jats:italic>Plasmodium</jats:italic> transmission together with vertical transmission and avirulence makes it a candidate for the development of a symbiont-based malaria transmission blocking strategy. We investigate the characteristics and efficiencies of <jats:italic>Microsporidia MB</jats:italic> transmission between <jats:italic>An. arabiensis</jats:italic> mosquitoes. We show that <jats:italic>Microsporidia MB</jats:italic> is not transmitted between larvae but is effectively transmitted horizontally between adult mosquitoes. Notably, <jats:italic>Microsporidia MB</jats:italic> was only found to be transmitted between male and female <jats:italic>An. arabiensis</jats:italic>, suggesting sexual horizontal transmission. In addition, <jats:italic>Microsporidia MB</jats:italic> cells were observed infecting the <jats:italic>An. arabiensis</jats:italic> ejaculatory duct. Female <jats:italic>An. arabiensis</jats:italic> that acquire <jats:italic>Microsporidia MB</jats:italic> horizontally are able to transmit the symbiont vertically to their offspring. We also investigate the possibility that <jats:italic>Microsporidia MB</jats:italic> can infect alternate hosts that live in the same habitats as their <jats:italic>An. arabiensis</jats:italic> hosts, but find no other non-anopheline hosts. Notably, <jats:italic>Microsporidia MB</jats:italic> infections were found in another primary malaria African vector, <jats:italic>Anopheles funestus s.s</jats:italic>. The finding that <jats:italic>Microsporidia MB</jats:italic> can be transmitted horizontally is relevant for the development of dissemination strategies to control malaria that are based on the targeted release of <jats:italic>Microsporidia MB</jats:italic> infected <jats:italic>Anopheles</jats:italic> mosquitoes.</jats:p>
