Individual starter culture were inoculated into liquid medium and incubated at $40^{circ}C$ for 16 hours. Whole cell were obtained and evaluated for ${eta}-galactosidase$ activity using orthonitrophenyl-${eta}-D-galactopyranoside$ (ONPG) as substrate. S. thermophilus had more ${eta}-galactosidase$ activity than other Lactobacilli did. To study the effect of storage temprature on enzyme activity of yoghurt, some samples of cultured yoghurt were stored under refrigeration $(4^{circ}C)$, and the others under room temperature $(23^{circ}C)$. At $4^{circ}C$, yoghurt had ${eta}-galactosidase$ activity and many viable bacteria in 1 month. After 20 days, yoghurt had maximum ${eta}-galactosidase$ activity. At $23^{circ}C$, yoghurt had ${eta}-galactosidase$ activity by 5 days. As this experiment shown ${eta}-galactosidase$ activity was ascribed to viable bacteria, especially S. thermophillus. Commercial yoghurt had lower ${eta}-galactosidase$ activity. There were considerable variations with regard to the lactose hydrolyzing capabilities of commercial yoghurt samples.