Microbial communities play an important role in stream ecosystem processes, such as breakdown of senescent leaf litter, and as a primary nutritional source for detritivorous macroinvertebrates. Antibiotics may affect stream microbial communities and associated ecosystem processes, especially because recent stream and river monitoring programs have indicated the presence of antibiotics downstream of wastewater treatment plants. In the current study, effects of chronic exposure to the fluoroquinolone antibiotic ciprofloxacin (Cipro) were examined on stream microbial community-level physiological profiles and growth indices of detritivorous amphipods (Gammarus spp.) and caddisflies (Lepidostoma liba). Microcosm experiments were conducted using stream sediments and water, senesced leaf material (Acer saccharum), and macroinvertebrates. A shift in function of leaf-associated microbial communities (based on carbon source utilization) was observed for samples exposed to 100 microg/L of Cipro for 12 d compared to control and treatments exposed to 1 and 10 microg/L of Cipro. This was attributable to carbohydrate substrates, which had 2.7- to 3.5-fold lower microbial respiration than the lower concentrations and control (p < 0.001). For detritivores, Gammarus spp. condition index did not differ among control, 0.1, and 1.0 microg/L treatments after 30-d exposures (p>> 0.05). Similarly, L. liba growth rate did not vary among control, 10, and 100 microg/L treatments after 45-d exposures (p>> 0.05). These results suggest that Cipro may affect leaf-associated microbial communities, but at concentrations four orders of magnitude above those detected in streams. However, effects of the antibiotic on growth and condition of detritivores were not observed. Future work should focus on identifying specific changes in stream microbial communities as a result of Cipro exposure and impacts on other aquatic species.