The spore density of arbuscular mycorrhizal fungi and mycorrhiza formation in soybean roots were surveyed in acid sulfate soils of soybean fields in Vietnam to estimate how arbuscular mycorrhizal fungi are involved in phosphate absorption. The survey was conducted in the Vi-Tan village in The Socialist Republic of Vietnam. Samples of the surface soil (to a depth of 10 cm) and soybean roots were collected on March 18, 2004. The pH of the surveyed soil was 3.9 and there were 34.1 spores/g dry soil. Arbuscules and vesicles of arbuscular mycorrhizal fungi were observed in soybean roots. Mycorrhizae formed on 30.1% of the sampled roots. The presence of arbuscular mycorrhizal fungi in soil with a pH less than 4 indicates that they are either acid soil adapted or are a species or strain that is acid resistant. In addition, the presence of significant numbers of mycorrhizae in soybean roots suggests that the fungi may be important in the absorption of phosphate in acid sulfate soils.