We have followed, in glucose-limited chemostats, the evolution of natural isolates of Escherichia coli possessing maximal growth rates of 0.48-1.43 doublings/h. Under these conditions a rapid-growth phenotype similar to that of standard laboratory wild-type strains was selected so that after 280 generations all of the cultures were characterized by bacteria with maximum growth rates close to 1.33 doublings/h. The growth yields of the natural isolates, on the other hand, were quite uniform and improved only slightly during the selection; it seems that the natural isolates are nearly maximally efficient at utilizing glucose. Some of the kinetic characteristics of ribosomes prepared from natural isolates vary markedly and in proportion to the growth rates of the original strains. After growth in glucose-limited chemostats, the ribosomes of all of the cultures become kinetically indistinguishable from those of laboratory wild-type bacteria. These observations confirm the interpretation that bacteria grown under normal laboratory conditions have been selected for maximum growth rates which demand maximum translation efficiency. In contrast, these characteristics do not seem to be strongly selected in the natural isolates.