Penicillin inhibited growth and killed growing cells of the moderate halophile Vibrio costicola. Cells were more susceptible to penicillin at lower than at higher NaCl concentrations. Inhibition of growth by KCN prevented the lethal action of penicillin. Envelopes of treated cells showed structural damage. Chains of cells separated under the action of the drug. Treated cells swelled rapidly, an unexpected reaction in a bacterium whose internal and external osmolarities are thought to be the same.