The prevalence of Bordetella bronchiseptica in the nasal cavity of pigs and the in-vitro sensitivity of isolates to a variety of antimicrobial agents was investigated. B. Bronchiseptica was recovered from 372 nasal swabs collected from 1000 (37.2%) pigs slaughtered at 20-30 weeks old at an abattoir. The swabs were collected from groups of 5-206 pigs derived from 25 herds. All isolates tested against bacitracin, clindamycin, furazolidone, penicillin, spectinomycin, streptomycin and tylosin were found to be resistant. Of the 372 isolates tested against ampicillin and erythromycin 22 (6%) were sensitive to the former and 365 (98%) were moderately sensitive to the latter, the remainder were resistant. All isolates tested against neomycin and tetracycline were sensitive and with few exceptions, (2%), they were also sensitive to chloramphenicol. Overall, 259 of the 372 (70%) isolates were sensitive to sulphonamides, identical results being obtained with sulphadiazine, sulphafurazole and a trimethoprim-sulphamethoxazole combination. An association between in-vitro resistance to sulphanomides and extensive use of this group of drugs was demonstrated on three of eight farms investigated.