110 El Tor Vibrio cholerae isolates from 102 patients with cholera between November, 1977, and March, 1978, during the early stages of the fourth epidemic of cholera in Tanzania had minimum inhibitory concentrations to tetracycline, chloramphenicol, nitrofurantoin, neomycin, ampicillin, and sulphadimidine determined. All isolates during the first month after the disease was recognised were fully sensitive to tetracycline, but 76% of isolates were resistant to the drug after five months of extensive use of tetracycline therapeutically and prophylactically in the country. Resistance to the five other antibacterial agents developed more slowly. Isolates from patients who failed to clear the organism from their stools or who had cholera soon after tetracycline prophylaxis had increased minimum inhibitory concentrations of the drug. Resistance did not develop in vivo. Although resistance to tetracycline readily developed following extensive use of the drug, such a resistance was not the only reason for failure of tetracycline treatment and prophylaxis. Mass chemoprophylaxis in the control of cholera should be discouraged unless evidence to the contrary becomes available.