To characterize the prevalence and heterogeneity of Neisseria gonorrhoeae with chromosomally mediated resistance to penicillin G or tetracycline.HCl in Seattle, Washington, and Denver, Colorado, we auxotyped, serotyped, and determined the MICs of penicillin G and tetracycline for gonococcal isolates collected in both cities during 1984. In Seattle 37 (18%) and ten (5%) of 205 isolates had MICs for penicillin G of greater than or equal to 1.0 and greater than or equal to 2.0 micrograms/ml, respectively; in Denver eight (3%) of 240 isolates had MICs for penicillin of 1.0 microgram/ml, and none had MICs of greater than 1.0 microgram/ml. For tetracycline.HCl, 107 (52%) and 38 (19%) of Seattle isolates had MICs of greater than or equal to 1.0 and greater than or equal to 2.0 micrograms/ml, respectively, while in Denver the respective figures were 111 (46%) and 48 (20%). In each city, antimicrobial resistance was present in a number of auxotype/serovar (A/S) classes: isolates with MICs for penicillin G of greater than or equal to 1.0 microgram/ml were identified in 15 of 49 A/S classes in Seattle and in five of 49 A/S classes in Denver. These data indicate that chromosomal resistance to penicillin G and tetracycline.HCl varies geographically in prevalence and is a heterogeneous phenomenon involving multiple gonococcal strains.