Processing alpha-glucosidase I, which is classified into glycosyl hydrolase (GH) family 63, hydrolyzes an oligosaccharide precursor of eukaryotic N-linked glycoproteins. Recently, many bacteria have been reported to possess genes for proteins that are homologous to the GH family 63 glucosidases. In this paper, Escherichia coli K12 YgjK protein, a member of GH family 63, was overexpressed, purified and crystallized using the vapour-diffusion method. Diffraction data were collected to 1.8 A resolution and the crystal was found to belong to the monoclinic space group P2(1), with unit-cell parameters a = 88.5, b = 137.1, c = 60.9 A, beta = 98.1 degrees. The V(M) value was determined to be 2.1 A(3) Da(-1), which corresponds to the presence of two protein molecules in the asymmetric unit.