Treatment of sinonasal bacterial biofilms continues to be a challenge in modern rhinology. This study’s objective was to assess safety and efficacy of topically applied bacteriophage alone and in combination with ethylenediaminetetraaceticacid (EDTA) for treatment of Staphylococcus aureus biofilms in vivo. Using a sheep model of sinusitis, frontal sinuses (n=6 per treatment) were flushed once daily with a S. aureus bacteriophage cocktail (2x106pfu/mL), with or without EDTA (0.075mg/mL), and compared to a control flush containing saline and heat-inactivated bacteriophage. Safety was assessed using histology and scanning electron microscopy (SEM) following treatment for 3 days. Efficacy was assessed by quantifying the generation of S. aureus biofilmsin the frontal sinuses. Biofilm mass was compared between treatment groups and controls using LIVE/DEAD BacLight staining and confocal scanning laser microscopy to visualize the tissue sections. COMSTAT2 software allowed computation of the biofilm mass present. Tissue morphology was conserved, with no significant signs of inflammation, when comparing control and test treatments. Further, SEM analysis indicated test treatments were not toxic or damaging to mucosal cilia. COMSTAT2 quantification of biofilm showed a significant reduction in biofilm levels when comparing the control with bacteriophage (p = 0.0043), EDTA (p = 0.0095) and bacteriophage-EDTA (p = 0.0022) treatments. Our results indicate bacteriophage and EDTA to be safe and efficacious for use topically against S. aureus infection in a sheep sinusitis model, and have potential to be translated to a clinical setting.