Agar dilution antimicrobial susceptibility testing showed that Campylobacter jejuni was significantly more resistant than Campylobacter fetus subsp. fetus (intestinalis) to cephalosporin C, cephaloridine, cephalothin, cefazolin, and cefamandole. No species differences in susceptibility were noted with cephalexin, cefotaxime, and cefoxitin. Rapid species differentiation on the basis of an antibiogram could be achieved with the disk diffusion method. C jejuni failed to produce a zone of inhibition around a 30-microgram cephalothin disk but produced a significant zone around a 30-microgram nalidixic acid disk. C. fetus subsp. fetus (intestinalis) produced exactly the reverse pattern.