[1] Large animal colonies are significant sources of atmospheric NH3 that can potentially increase the nitrogen status of nearby ecosystems. Atmospheric NH3 concentrations were measured at different distances from a Cape fur seal colony at Cape Cross, northwest coast of Namibia, and an atmospheric dispersion model was applied inversely to estimate the total NH3 emission. Mean concentrations ranged from 354 to 0.01 μg NH3 m−3 at 0.01 and 26 km from the source, respectively. The model simulations suggest that the colony emits 25–70 tonnes of NH3-N yr−1, c. 3% of the total N excreted by the seals.