Abstract Four Escherichia coli mutants with defects in the α subunit of H +-ATPase (F 0F 1) (strain KF154, Pro-281 → Leu; KF101 and KF131, Ala-285 → Val; KF114, Arg-376 → Cys) were isolated, and the kinetic properties of their F 1-ATPases were studied. All the mutations so far identified are clustered in the two defined regions of the α subunit. With F 1 of strain KF114, as with F 1 of uncA401 (Ser-373 → Phe; T. Noumi, M. Futai, and H. Kanazawa (1984) J. Biol. Chem. 259, 10076–10079) , the rate of multisite hydrolysis of ATP was 4 × 10 −3-fold lower than that with wild-type F 1; suggesting that residues Ser-373 and Arg-376 or the regions in their vicinities are essential for positive catalytic cooperativity. With F 1 from strain KF101, multisite hydrolysis was higher (about 40% of that of the wild type), but the F 1 was unstable and showed defective interaction with the membrane sector (F 0). The F 1 from KF154 had lower multisite hydrolysis (about 10% of that of the wild type) but could support slow growth by oxidative phosphorylation.