The mycorrhizae and respiration of roots of Salix nigra Marsh., were investigated for plants growing on an abandoned railroad grade in Virginia. The plants were found to be heavily colonized with vesicular–arbuscular mycorrhizae when growing on this site. Glomus fasciculatus (Thaxter) Gerd. & Trappe was the only symbiont which could be unequivocally connected with these endomycorrhizal roots.Addition of 5.0 mM KCN to S. nigra endomycorrhizae or S. rotundifolia ectomycorrhizae inhibited respiration by approximately 50%, suggesting the possible involvement of a cyanide insensitive alternate oxidase in root respiration. Addition of 2.5 mM salicylhydroxamic acid (SHAM) to KCN inhibited roots caused an additional significant reduction of oxygen uptake in both root types. These results suggest the participation of a SHAM sensitive oxidase in the respiration of these roots. When SHAM was added alone to S. nigra endomycorrhizae a slight but insignificant reduction in oxygen uptake was observed. Conversely, SHAM caused a stimulation of oxygen uptake in S. rotundifolia ectomycorrhizae. When SHAM was followed by the addition of KCN the net result was a significant inhibition of respiration in both types of roots. These results indicate that an alternate oxidase exists in these endo- and ectomycorrhizal roots of willows which is characterized by some of the same basic features described for beech ectomycorrhizal roots.