The penicillin-binding protein 2 genes (penA) of penicillin-resistant Neisseria meningitidis have a mosaic structure that has arisen by the introduction of regions from the penA genes of Neisseria flavescens or Neisseria cinerea. Chromosomal DNA from both N. cinerea and N. flavescens could transform a penicillin-susceptible isolate of N. meningitidis to increased resistance to penicillin. With N. flavescens DNA, transformation to resistance was accompanied by the introduction of the N. flavescens penA gene, providing a laboratory demonstration of the interspecies recombinational events that we believe underlie the development of penicillin resistance in many meningococci in nature. Surprisingly, with N. cinerea DNA, the penicillin-resistant transformants did not obtain the N. cinerea penA gene. However, the region of the penA gene derived from N. cinerea in N. meningitidis K196 contained an extra codon (Asp-345A) which was not found in any of the four N. cinerea isolates that we examined and which is known to result in a decrease in the affinity of PBP 2 in gonococci.