SUMMARYThe percentage colonization of roots by arbuscular mycorrhizal fungi was measured in the field in two crops of spring oats {Avena sativa L.) and one of maincrop potatoes {Solanum tuberosum L.), successively grown over a period of 3 yr on the long-term liming experiments at Rothamsted and Woburn. In both crops, all years, both phosphorus treatments and at both sites, percentage colonization was little affected by soil pH from over a range approximately 4-5 to 7-5. There was little effect of pH on crop yields. Up to nine species of coarse endophyte could be identified from spores (> 50 //m diameter) m plots of pH 55 and above. There were markedly fewer spores at (approximately) pH 5-5 and none in the most acid plots.