none This study explores the extra-host interactions between the braconid endoparasitoid, Apanteles glomeratus , and the granulosis virus (GV) of Pieris brassicae , focusing on factors influencing the transmission of the virus within and between host broods. Transmission on the virus by A. glomeratus adult females was shown to correlate positively with the time since death of the virus-killed donor larvae. Transmission was most likely to occur from donors which were dead for 24-48 h. In the absence of the parasitoid, the risk of infection to a given P. brassicae larva was dependent on (i) the host age at the time of the introduction of the virus, and (ii) the initial number of the larvae infected. The initial number of uninfected hosts, however, had no effect on the per capita risk of infection. The parasitoid accelerated the spread of the GV in the host brood, but the eventual number of hosts succumbing to the infection was unaffected.