Aim: External fixation surgery is frequently complicated by percutaneous pin site infection focused on the surface of the fixator pin. The primary aim of this study was to compare biofilm growth of clinically isolated pin site bacteria on Kirschner wires of different materials. Materials and methods: Two commonly infecting species, Staphylococcus epidermidis and Proteus mirabilis, were isolated from patients’ pin sites. A stirred batch bioreactor was used to grow these bacteria as single culture and co-cultured biofilms on Kirschner wires made of three different materials: stainless steel, hydroxyapatite-coated steel and titanium alloy. Results: We found that the surface density of viable cells within these biofilms was 3x higher on stainless steel and 4.5x higher on hydroxyapatitecoated wires than on the titanium wires. Conclusion: Our results suggest that the lower rates of clinical pin site infection seen with titanium Kirschner wires are due to, at least in part, titanium’s better bacterial biofilm resistance. Clinical significance: Our results are consistent with clinical studies which have found that pin site infection rates are reduced by the use of titanium relative to stainless steel or hydroxyapatite-coated pins.