The anaerobic digestion of outdoor cultivated Rhizoclonium biomass was investigated in this study. The influence of the pre-treatment of the biomass using mechanical and biological methods on the overall methane yields was studied. The results show that the application of an enzymatic pre-treatment improved the methane yields by > 20% compared to the mechanical methods. The methane yields from Rhizoclonium biomass was however observed to be considerably lower than those of other algae species.