Arbuscular mycorrhizal fungi (AMF) show significant potential for biocontrol of Phytophthora spp., but there is little evidence for the mechanisms involved in the process. This study establishes that microorganism free exudates from roots colonised by AMF result in significantly less sporulation of P. fragariae than those from uncolonised plants. Experiments in vitro showed that after 48 h in the presence of exudates from strawberry roots colonised by Glomus etunicatum and G. monosporum, sporulation of P. fragariae was reduced by ca 67% and 64% relative to sporulation in the presence of uncolonised roots. After 72 h sporulation was reduced by 83% and 89% respectively. These data were then confirmed in an in vivo system in which Phytophthora fragariae was inoculated into the mycorrhizosphere of either uncolonised strawberry plants or those colonised by G. etunicatum. A similar trend was observed, with a 69% reduction in sporulation of P. fragariae after 72 h in the mycorrhizosphere of colonised plants relative to sporulation in the mycorrhizosphere of uncolonised plants.