A screening for microorganisms able to metabolize insoluble plant cell wall constituents was performed. As plant material, grass (Poaceae) was chosen. The selectivity of the screening medium was increased by previously extracting soluble sugars and proteins with 1 M phosphate buffer, pH 7. About 45 bacterial and 44 fungal strains were isolated. A gravimetrical method had to be optimized to determine microbial growth and the grass cell wall material converted, since conventional methods couldn't be applied due to interference of the grass particles. The selected fungi are the most promising group for the degradation of insoluble plant cell wall polymers; up till 30% conversion within 7 days was observed with the best fungal strains.