Filamentous green algae, Cladophora glomerata, obtained from rivers and streams in California was used as a filler in isocyanate-based foams. Up to 40 wt % dried and chopped algal fibers were added to the isocyanate-polyol mix. The algal fibers and foam samples were examined by scanning electron microscopy. The effective hydroxyl number of the algae and the tensile and compressive properties of the composite foam were measured. The results indicate that foams with a uniform cell size were obtained for algal fiber lengths between 500 and 600 mum. Peak mechanical properties were obtained for algae concentrations between 5 and 10 wt %. The elastic modulus of the foam follows a power law relationship with density, with an exponent of 1.6. The overall hydroxyl number for the algae is around 300. The hydroxyl groups in the algae can serve as efficient sites for reactions with isocyanate and hence algae fibers can act as reinforcing fillers. (C) 2004 Wiley Periodicals, Inc.