Two experiments were set up to investigate the influence of soil organic matter on growth of arbuscular mycorrhizal (AM) hyphae and concurrent changes in soil inorganic P, organic P and phosphatase activity. A sandy loam soil was kept for 14 months under two regimes (outdoor where surplus precipitation leached through the soil, or indoor at constant moisture) with or without 9% (w/w) chopped wheat straw plus mineral N. Then the soils were partially sterilized and placed in two-compartment pots where mycorrhizal or non-mycorrhizal cucumber plants were grown in one root compartment (RC), and soils differing in organic matter were placed in six parallel hyphal compartments (HC) separated from the RC with a 37 mu m mesh. In the first experiment, using Glomus caledonium, hyphal length densities were measured in the HC after 31 days. Added straw increased hyphal length densities by 34 and 62% for soil kept outdoors and indoors, respectively. In the second experiment, using G. invermaium and only soil kept outdoors, three treatments were included: soil with no added straw with or without a new addition of 0.5% (w/w) of ground clover leaves, and soil with 9% straw plus mineral N. After 41 days hyphal length density was twice as high in soil with added straw compared to the two other treatments. Mycorrhizal colonization resulted in lower activity of acid phosphatase in the HC for two out of three treatments. Alkaline phosphatase activity was only decreased by mycorrhiza in soil without organic matter additions. In soil with added clover alkaline phosphatase activity increased due to the presence of mycorrhizal hyphae. We suggest that mycorrhizas may influence the exudation of acid phosphatase by roots. Hyphae of G. invermaium did apparently not excrete extracellular phosphatases, but their presence may have influenced alkaline phosphatase excreted by other microorganisms, probably through competition for nutrients. Phosphatase activity was not correlated with the concentration of labile organic P in soil extracts.