We investigated the influence of changes in external phosphorus (P) concentration on the proportion of phosphatase-active structures of the arbuscular mycorrhizal fungus Gigaspora margarita associated with Allium cepa. The P treatment was started when mycorrhizal colonisation had been established, and plant systems were harvested three times after the start of the P treatment. Higher shoot dry weights and P contents were observed in the high-P treated plants at the last harvest. We did not find any change in the proportion of phosphatase-active extraradical mycelium following P treatment. However, the proportion of alkaline phosphatase-active mycelium was positively correlated for extraradical and intraradical mycelium. Also, the proportion of alkaline phosphatase-active arbuscules seemed to increase with the shoot fresh weight, whereas the proportion of acid phosphatase-active arbuscules decreased with higher shoot P concentration and dry weight. We have shown experimentally that the intraradical mycelium of G. margarita, but not the extraradical mycelium, responds metabolically to plant P concentration, and possibly also to external P availability.