We prospectively compared the clinical features of cholera due to Vibrio cholerae O1 and V. cholerae O139 in 242 men 18-60 years of age, with a history of diarrhoea of 24 h or less, and moderate or severe dehydration. The antimicrobial susceptibility of all of the V. cholerae strains isolated from these patients was determined, and in vitro cholera toxin production determined for 68 isolates. On admission, the 110 patients infected with V. cholerae O1 significantly more often had body temperature < 36°C (85% vs. 66%, P ≤ 0.05), faecal leucocyte count > 50/high power microscope field (40% vs. 12%), and lower mean faecal chloride content (94 vs. 103 mmol/L) than did the 132 patients infected with V. cholerae O139. Patients infected with V. cholerae O1 also initially had significantly higher median volumes of stool (13 vs. 11 mL per kg body weight per h), vomitus (1 ml/kg/h vs. nil), and intravenous fluid requirements (23 vs. 21 mL/kg/h). All V. cholerae O1 and O139 isolates were susceptible to ciprofloxacin, all but one were susceptible to doxycycline and erythromycin, and the majority of both serogroups were resistant to co-trimoxazole (95% and 97%, respectively). V. cholerae O1 and O139 susceptibilities differed for tetracycline (58% vs. 100%) and furazolidone (27% vs. 93%) (P < 0.001 in both cases). The amount of cholera toxin produced in vitro by strains of V. cholerae O1 and O139 was similar, and did not correlate with stool volume. The results demonstrated that V. cholerae O139 does not cause more severe, or more invasive, disease than V. cholerae O1, as had been previously suggested, but that clinically important differences in antimicrobial susceptibility do exist among strains isolated in Bangladesh.