The impact of Glomus fasciculatum on Meloidogyne hapla associated with Allium cepa was evaluated in two experiments. Nematode density was not different in mycorrhizal and nonmycorrhizal plants 10 weeks after the joint inoculation of M. hapla and G. fasciculatum. Differences in the age structure of M. hapla populations reared on mycorrhizal and nonmycorrhizat plants were noted. G. fasciculatum enhanced leaf and bulb growth of A. cepa in the absence of M. hapla, but did not affect plant weight when nematodes were present. Survival and reproduction of M. hapla were not affected by G. fasciculatum or phosphorus (P). The estimated time required for inoculated second-stage juveniles (J2) to mature to the adult stage was 1,000 degree hours (base = 9 C) greater in mycorrhizal than in nonmycorrhizal plants supplemented with P. Although the infectivity of J2 was not measured directly, colonization of A. cepa by G. fasciculatum appeared to alter the ability of M. hapla to penetrate roots.