In this study, a pilot-scale plant was built to examine the practicality of producing biogas from seaweeds, widely available in Japan. Laminaria sp. and Viva sp. seaweeds were mixed with other organic waste (milk) and used as fermentation materials. Though quantities and ratios of the materials were varied, the ratio of generated methane to input chemical oxygen demand (COD) was largely stable (0.2-0.3 m(3) methane/kg COD) and the organic acid concentration in the methane fermentation solution was low (<1200 ppm) during prolonged operation. These findings indicate that stable methane fermentation was achieved and that mixing with other organic material was effective in suppressing fluctuations in material amounts caused by the variable supply of seaweeds. Our results demonstrate the practical feasibility of biogas generation using seaweeds. (C) 2010, The Society for Biotechnology, Japan. All rights reserved.