The plasmid content of one penicillin sensitive and six penicillin resistant strains of Neisseria gonorrhoeae has been examined. All strains harbour a small, phenotypically cryptic plasmid of 4.1 kilo base pairs (kb). Four of the penicillin resistant strains carry a beta-lactamase-producing plasmid of 7.3 kb. One of these also carries a large plasmid of about 40 kb. The two remaining penicillin resistant strains harbour a smaller beta-lactamase-producing plasmid of 5.5 kb. The plasmids have been subjected to digestion with a number of restriction endonucleases, and their restriction maps have been compared. Judging by the maps, the cryptic (C-) plasmids show great similarities. Except that two of them have 54 additional base-pairs (bp), and two have a HpaII site instead of a DdeI site, no differences were found. The larger beta-lactamase-producing (B-) plasmids have identical maps. The smaller seem to be homologous with the larger, except for a deletion of 1.8 kb. There is no correlation between the variant of C-plasmid and type of B-plasmid harboured in the penicillin resistant strains. The evolutionary implications suggested by this finding are discussed.