In the perfusion culture of mammalian cells, the temperature T_v that maximized the viable cell density at steady state was lower by 6-8°C conpared to temperatureT_g that maximized cell growth rate. The cell density was 2.4 times as high at T_v as at T_g. Since the death rate decreased as the temperature was lowered, the difference between the growth and death rates could be large enough to maximize the viable cell density, the difference integrated with culture time. Protein production rate per cell as well as per culture volume was maximized at a temperature several degree higher than T_v.