In this work we investigated the hypothesis, suggested by unpublished experiments carried out previously, that application of MES [2-(N-morpholino)-ethane sulphonic acid] buffer to the substrate of plants hosting the arbuscular mycorrhizal fungus Glomus intraradices might favour the growth of external hyphae. The substrate of Trifolium pratense plants, with or without G. intraradices inoculum, was treated twice weekly, between weeks 5 and 11 of growth, plant with distilled water, P-supplemented 10% Hewitt nutrient solution (H+P), 51mM in H+P or 1mM TRIS-HCl in H+P. MES buffer increased the length of external G. intraradices hyphae and the density of propagules in the substrate, but plant growth was not affected. Although both TRIS-HCl and MES caused substrate pH to fall (from 8.51 to 7.35 and 7.17, respectively), only MES induced a significant increase in external hypha length. We conclude that the effect of MES on external hypha length was not due to plant growth or to substrate pH per se.