Fixation of atmospheric nitrogen by bacterial symbionts of plants affects not just plant growth, dynamics, and ecosystem properties, but also biotic interactions. However, direct impacts of biotic interactions on N-fixation are largely unknown, because most studies of variation in N-fixation focus on abiotic factors, including soil propertiess (nitrogen, phosphorus or water contents), light, and temperature. We assessed impacts of browsers (deer) on N-fixation by a shrub, Ceanothus cuneatus rigidus, and its actinorhizal symbiont, Frankia, to test the hypothesis that heavy browsing reduces N-fixation, presumably because loss of photosynthetic tissue reduces net carbon fixation and the plant’s contributions to the symbiosis. We examined stable N and C isotope compositions of leaves from Ceanothus and adjacent non-fixing shrubs, Arctostaphylos pumila, in a long-term field experiment that excluded browsers from some plants. Heavily browsed Ceanothus had the same δ¹⁵N values as adjacent non-fixing Arctostaphylos, and we conclude that Ceanothus was not fixing atmospheric N₂ under natural browsing levels. In contrast, within a year after deer browsing was prevented, δ¹⁵N values were close to 0‰, showing that the Ceanothus/Frankia symbiosis had begun to fix atmospheric N₂; these levels were maintained for at least 7 years without browsing. δ¹³C values did not change over time in browsed or unbrowsed Arctostaphylos or in browsed Ceanothus, but δ¹³C values increased in unbrowsed Ceanothus, which also grew faster than browsed controls. The higher δ¹³C values may reflect either higher photosynthetic rates or greater water limitation in N-fixing plants.