A pot experiment was carried out with the following treatments: non-mycorrhizal control and inoculation by arbuscular mycor-rhizal fungi (Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe or Sclerocystis sinuosa Gerd. & Bakshi) at four P levels (0, 9.5, 19, and 38 mg P kg(-1)). Cotton (Gossypium arboreum L. cv. Xin-lu-zao No.1) was used as the host plant. The N-15-labeled urea (2.5% abundance) was mixed with soil thoroughly at a rate of 100 mg Nkg(-1). Colonization by the two arbuscular mycorrhizal fungi significantly improved the growth of cotton plants at all P levels compared with non-mycorrhizal plants. Nitrogen concentration in shoots was not significantly affected by arbuscular mycorrhizal fungi, while the nitrogen content was significantly higher in mycorrhizal plants than in non-mycorrhizal plants. The N-15 atom % excess in mycorrhizal plant was significant lower than that in non-mycorrhizal plants. Considering the derivation of the nitrogen in the cotton plants, mycorrhizal fungi had no significant influence on uptake of nitrogen derived from fertilizer, but increased nitrogen derived from soil significantly. Colonization by mycorrhizal fungi increased the amount of soil available nitrogen ("A(N)" value). The results indicate that arbuscular mycorrhizal fungi may facilitated plant acquisition of nitrogen from sources which are not or less available to non-mycorrhizal plants.