The purpose of this research was to investigate the effects of low concentration of oxygen on reduction of perchlorate, especially low perchlorate influent concentrations in a biofilm reactor, as well as the effect of flow pattern in a biofilm reactor. Dissolved oxygen averaging 1 mg/L did not inhibit reduction of influent perchlorate from 23 to $426;{mu}g/L$ in the biofilm reactors when sufficient acetate was added, probably due to limitation of oxygen diffusion into the biofilm. Influent perchlorate ranging from 23 to $426;{mu}g/L$ was reduced to below detection level ($4;{mu}g/L$) in the presence of 1 mg/L dissolved oxygen (DO). Chloride was produced in a ratio of $0.37gCl^-/g{ClO_4}^-$ and $0.35gCl^-/g{ClO_4}^-$ in plug flow and recirculation biofilm reactor which is similar to stoichiometric amount ($0.36gCl^-/g{ClO_4}^-$) indicating complete perchlorate reduction at $426;{mu}g/L$ of ${ClO_4}^-$ feeding. At $23;{mu}g/L$L influent perchlorate, total biomass solids were 3.18 g and 2.81 g in the plug flow and recirculation biofilm reactors. The most probable number(MPN) analysis for perchlorate-reducing bacteria showed $10^4$ to $10^5;cells/cm^2$ in both biofilm reactors throughout the experiments. The effluent perchlorate concentrations were not significantly different in the two different flow regimes, plug flow and recirculation biofilm reactors.