Previous research showed that during intermediate stages of primary succession, when vegetation is dominated by ectomycorrhizal (EcM) shrubs and trees, site colonization by earthworms substantially alters plant communities. Research has also shown that EcM shrubs and trees suppress arbuscular mycorrhizal (AM) plants in the understory. To determine whether earthworm activity reduces this asymmetric competition, we conducted a full factorial laboratory experiment in which we grew EcM Betula pendula and AM Tripleurospermum inodorum, together or apart, in soils affected or not affected by earthworms. When both plants were grown together in soil unaffected by earthworms, growth of T. inodorum was significantly reduced by competition with B. pendula, but B. pendula growth was not reduced by T. inodorum. In soil affected by earthworms, the growth of both species was increased, and the negative effect of B. pendula on T. inodorum was no longer statistically significant (P < 0.05). These data indicate that earthworms weaken the asymmetric competition between EcM and AM plants. Consistent with this inference, EcM colonization of B. pendula was decreased and AM fungal bioassay in soil was increased by earthworms.