AbstractFive extremely thermophilic cellulose‐degrading isolates obtained from New Zealand thermal springs were tested for their ability to degrade a number of natural lignocellulosic substrates. Degradation by three of the isolates was generally similar to that by the moderate thermophile, Clostridium thermocellum but occurred at a higher temperature. The New Zealand isolates were also found to grow on xylan as sole carbohydrate source, which probably extends their attack to hemicellulose.