The endoglucanase Ce1A from Clostridium thermocellum was strongly expressed in Bacillus subtilis. The enzyme was purified by Ni2+ -affinity chromatography. Site-directed substitution of D278 with an asparagine or an alanine residue surprisingly did not decrease the apparent k(cat) value. Further substitutions of two other potentially critical residues, Y215 and D152, resulted in a 2-fold decrease in apparent k(cat) value for Y215P and complete loss of activity for D152N.