The enzyme displays high hydrolysis ability for ONPG (100%) and moderate activity for its natural substrate lactose (25.7%). However, the hydrolysis ability of the enzyme towards all other chromogenic nitrophenyl analogues is very weak, indicating that Gal308 is a β-galactosidase with narrow substrate specificity. To investigate the kinetic parameters of recombinant enzyme, the Michaelis-Menten constants (K _m ), turnover numbers (k _cat ), and catalytic efficiencies (k _cat /K _m ) of Gal308 for ONPG and lactose are determined. The k _cat and K _m values are 464.7±7.8 s ^-1 and 2.7±0.3 mM for ONPG, and 264.2±2.1 s ^-1 and 7.1±0.8 mM for lactose, respectively. The k _cat /K _m value of the enzyme for ONPG (172.1 s ^-1 mM ^-1 ) is 4.6-fold higher than that for lactose (37.2 s ^-1 mM ^-1 ), which clearly demonstrated that the catalytic efficiency of Gal308 for ONPG is much higher than that for lactose.