A X-Gal test paper method for detecting β-galactosidase has been developed.This method is based on the degradation of 5-bromo-4-chloro-3-indolyl-β-galactoside,a chromogenic substrate, into blue 5-bromo-4-chloro-indole by the reaction of β-galactosidase. Advantage of the method is that it not only consums X-gal less, but it is also more simple and convenient than the previous method.The present method is suitable for detecting a large number of the colonies containing gene encoding for β-galactosidase from an agar plate.