The human respiratory tract pathogen Moraxella catarrhalis is a naturally competent microorganism. However, electrotransformation has long been used to introduce foreign DNA into this organism. This study demonstrated that electrotransformants obtained with linear or circular nonreplicating plasmid DNA originated exclusively from natural transformation processes taking place during the recovery phase after the application of current. Only replicating plasmid DNA could be introduced into M. catarrhalis by electrotransformation, in a type IV pilus-independent manner. Electrotransformation with homologous genomic DNA indicated that restriction of double-stranded DNA was independent of type III restriction-methylation systems. Nontransformability of M. catarrhalis by electrotransformation was observed using double- as well as single-stranded DNA. In addition, the study showed that natural competence is a very constant feature of M. catarrhalis.