Tetracycline and its derivatives are extensively used human and animal antibiotics, and enter stream ecosystems via point and non-point sources.   Laboratory studies indicate that microbial organisms are more sensitive to antibiotics than invertebrates or fish, and may indicate the presence of antibiotics in aquatic ecosystems.  We examined the consequences of tetracycline exposure to a periphyton community across a logarithmically dosed-series of experimental mesocosms receiving water from the East Fork of the Little Miami River in Milford, OH.  Mesocosms were colonized naturally, dosed, and then allowed to recover, each period lasting 30 days.  Target in-stream tetracycline exposures were based upon previously reported concentrations in streams and rivers throughout the United States (<1 to 100 µg L^-1), and included a control.  Our results indicated significant changes in the periphyton community with in-stream tetracycline concentrations as low as 0.5 µg L^-1, and effects magnified in proportion with increased dosing rate, including decreases in abundance of bacteria and algae, decreased bacteria productivity, and algal compositional shifts.  Recovery of the periphyton community ranged from days to several weeks depending on the organism and dosing concentration. These results suggest the potential for shifts in the trophic structure of streams after sustained (days) exposure to antibiotics.