This paper reports the characterization of clinical Vibrio cholerae resistant to vibriostatic agent O/129, using classical and plasmid analysis. In a study conducted during December 1991-September 1993, two of 7,058 V. cholerae strains, obtained from patients suspected to have cholera in the State of Ceará, northeast Brazil, were resistant to 150 micrograms of the vibriostatic agent O/129 (2,4-diamino-6,7-diisopropylpteridine). One strain was identified as V. cholerae O1 El Tor Inaba and the other one as serogroup O22. Only one O1 strain harboured a plasmid of 147 kb transferable to Escherichia coli K12, and five strains of V. cholerae O1 and non-O1 were sensitive to O/129 and plasmid-negative at a frequency between 8 x 10(-2) and 3.6 x 10(-5). Additionally, O/129-resistant strains of V. cholerae O1 and O22 were resistant to trimethoprim/sulphamethoxazole.