Significant differences among the three Providencia species (P. alcalifaciens, P. stuartii, and P. rettgeri) in antimicrobial susceptibilities to five antibiotics were shown. P. stuartii was the most resistant of the three species, and P. alcalifaciens was the most susceptible. P. rettgeri was similar to P. stuartii in susceptibilities to cefoxitin, cephalothin, and cefamandole but differed in showing greater susceptibilities to tobramycin and gentamicin. Cefoxitin (16 micrograms/ml) and cefamandole (16 micrograms/ml) inhibited a greater proportion of P. stuartii isolates than did cephalothin, tobramycin, or gentamicin. The susceptibilities of urea-positive isolates of P. stuartii resembled more closely the susceptibilities of urea-negative isolates of this species than those of P. rettgeri isolates, a finding consistent with the recent recommendation for transferring such urea-positive strains to P. stuartii. Among P. stuartii isolates, marked resistance to cefoxitin accompanied by susceptibility to cefamandole was predominantly restricted to isolates of one serotype (O55). The use of isolates that had been serotyped and classified according to recent proposals for taxonomic changes in the Proteeae provided for clearer demonstration of species differences in susceptibility.