The ability of four species of vesicular–arbuscular mycorrhizal (VAM) fungi to increase phosphorus uptake and growth of clover plants (Trifolium subterraneum L.) at different levels of soil compaction and P application was studied in a pot experiment. Dry matter in the shoots and roots of clover plants decreased with increasing soil compaction. Colonization by Glomus intraradices Schenck & Smith and Glomus sp. City Beach WUM16 increased plant growth and P uptake up to a bulk density of 1·60 Mg m−3, although the response was smaller as soil compaction was increased. Glomus etunicatum Becker & Gerdeman and Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe had no effect on the shoot d. wt and P uptake when the bulk density of the soil was [ges ]1·40 and [ges ]1·60 Mg m−3, respectively. Soil compaction to a bulk density of 1·60 Mg m−3 had no effect on the percentage of root length colonized by G. intraradices and Glomus sp. City Beach, but total root length colonized decreased as soil compaction was increased. Decreased P uptake and growth of clover plants colonized by G. intraradices and Glomus sp. City Beach, with increasing soil compaction up to a bulk density of 1·60 Mg m−3, was mainly attributed to a significant reduction in total root length colonized and in the hyphal biomass. Soil compaction, which increased bulk density from 1·20 to 1·75 Mg m−3, reduced the O2 content of the soil atmosphere from 0·16 to 0·05 m3 m−3. The absence of any observable mycorrhizal growth response to any of the four species of VAM fungi in highly compacted soil (bulk density = 1·75 Mg m−3) was attributed to the significant decrease in the O2 content of the soil atmosphere, change in soil pore size distribution and, presumably, to ethylene production.