Incomplete source budgets for methyl halides – compounds that release inorganic chlorine and bromine radicals which, in turn, catalyze atmospheric ozone depletion – limit our ability to predict the fate of the stratospheric ozone layer. We report here the first measured emissions of methyl chloride, methyl bromide, and methyl iodide from ectomycorrhizal fungi. We grew nine fungal isolates on growth media containing halide concentrations similar to those found in soils and plant tissues. The observed range of emissions was 0.003–65 μg methyl chloride, 0.001–3 μg methyl bromide, and 0.02–12 μg methyl iodide g−1 dry weight fungi day−1. Species varied in production rates of methyl chloride vs. methyl bromide vs. methyl iodide. Cenococcum geophilum, a widespread ectomycorrhizal fungus, was further tested to investigate the effects of halide substrate concentration in growth media. Emissions from this species increased linearly with increasing concentrations of both bromide and iodide. In addition, a subset of four fungi was studied with two media concentrations each of chloride, bromide, and iodide (0.2 or 20 mm). These fungi had similar responses to halide concentration, despite 1000-fold differences in baseline emission rates between isolates. Finally, high chloride concentrations (20 mm) in media did not appear to inhibit emissions of methyl bromide or methyl iodide. Overall, ectomycorrhizal fungi might be an important source of methyl halides to the atmosphere, and substrate concentrations and community composition may influence production levels in ecosystems.