This study investigated the effect of a rapid change in the concentration of the soil solution on hyphal growth from germinated spores of three arbuscular mycorrhizal fungi: isolates of Acaulospora laevis Gerd. & Trappe, Gigaspora decipiens Hall & Abbott, and Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders. Spores of either G. decipiens or S. calospora were incubated between millipore filters buried in sand that had been watered to field capacity with solutions of 0, 150, or 300 mmol/L NaCl. After 11 d, the intact pairs of filters were recovered, left undisturbed, or transferred into and further incubated in soil watered with one of the three solutions. Spores of A. laevis were incubated between filters in sand without NaCl and after 20 d were either left undisturbed or transferred to soil with 0, 50, 150, or 300 mmol/L NaCl in the soil solution for a further 11 d. The filter sandwiches were stained and opened, and determinations of spore germination, number of auxiliary cells, and length of hyphae on each were made. For G. decipiens and S. calospora, the effect of NaCl on hyphal growth was reversible. Hyphae from spores germinated in sand with 300 mmol/L NaCl showed markedly increased growth when transferred to a less saline environment. Hyphae from spores germinated in nonsaline sand continued to grow, but at a slower rate, when transferred to a saline environment. Hyphae of A. laevis continued to elongate after transfer to soil with 50 mmol/L NaCl but not 150 or 300 mmol/L NaCl. Morphological differences were observed between hyphae of G. decipiens grown in a highly saline as compared with a nonsaline substrate.