Nongrowing bacteria evade the bactericidal activity of βbT-lactam antibiotics. We sought to determine if slow growth rate also alters bactericidal activity. The bactericidal activity of two βbT-lactams on grown in glucose limited chemostats was compared for generation times ranging from 0·7 to 12 h. The degree of killing varied with drug structure and with strain. However, all killing rates were a constant function of the bacterial generation time: slowly growing bacteria became progressively more phenotypically tolerant to βbT-lactam antibiotics as the generation time was extended.