Road salt run-off causes episodic increases in salinity in streams that can be detrimental to aquatic biota. Stream biofilms play an important role in nutrient and carbon dynamics, and their activity affects the export of nutrients and carbon to downstream waterways. We conducted mesocosm experiments to investigate the effects of increased salt levels on stream biofilm activity. We exposed biofilms from four different streams to four levels of salt and measured biofilm activity as the decrease in dissolved organic carbon (= food source) over a three week period. We found that high salt concentrations depressed biofilm activity and that streams with lower ambient salt concentrations were depressed more than streams with higher ambient salt concentrations. Our results indicate that salt has both short and long term impacts on stream biofilm activity and may affect carbon and nutrient export from streams.