An endomycorrhizal fungus, Gigaspora margarita, was more effective in stimulating the growth of cotton (Gossypium hirsutum) 'Coker 201' at a low fertility level (1.77 gm 10-10-10 N-P-K/pot) than doubling the fertility rate for nonmycorrhizal plants. Gigaspora margarita alone stimulated shoot growth (height, weight, and flower production by 96%, 553%, and 760%, respectively) and root growth (385%) over that of nonmycorrhizal controls at low fertility. Plant development was also stimulated by G. margarita at the high fertility level (3.54 gm 10-10-10 N-P-K/pot), but the magnitude of the increase was not as great as that at the low fertility level. Although cotton was a suitable host for Pratylenchus brachyurus, plant development was not retarded by this nematode at either fertility level. In concomitant culture, mycorrhizal-induced plant growth, and sporulation of the endomycorrhizal fungus were not affected by P. brachyurus. Reproduction of P. brachyurus was similar on mycorrhizal and nonmycorrhizal cotton.