A psychrotrophic bacterium, Arthrobacter sp. ON14, isolated from Antarctica, was shown to exhibit a high ${eta}$-galactosidase activity at a low temperature. A genomic library of ON14 was constructed and screened for ${eta}$-galactosidase genes on functional plates containing 5-bromo-4-chloro-3-indolyl-${eta}$-D-galactopyranoside (X-gal) as the substrate. Two different ${eta}$-galactosidase genes, named as galA, galB, were found in ON14. Computational analyses of the genes revealed that the encoded protein GalA belongs to family 2 of glycosyl hydrolysases and is a cold-active protein, whereas GalB belongs to family 42 of glycosyl hydrolysases and is a mesophilic protein. Reverse transcription analyses revealed that the expression of galA is highly induced at a low temperature ($4^{circ}C$ ) and repressed at a high temperature ($28^{circ}C$ ) when lactose is used as the sole carbon source. Conversely, the expression of galB is inhibited at a low temperature and induced at a high temperature. The purified GalA showed its peak activity at $15^{circ}C$ and pH 8. The mineral ions $Na^+$, $K^+$, $Mg^{2+}$, and $Mn^{2+}$ were identified as enzyme activators, whereas $Ca^{2+}$ had no influence on the enzyme activity. An enzyme stability assay revealed that the activity of GalA is significantly decreased when it is incubated at $45^{circ}C$ for 2 h, and all its activity is lost when it is incubated at $50^{circ}C$.