Optimum conditions were defined for the electrotransformation of Pseudomonas aeruginosa PAO1 with plasmid pLAFR1, resulting in a 1500-fold increase in transformation efficiency compared to conventional chemical transformation with MgCl2. In addition, PAO236 and two out of three recent clinical isolates of P. aeruginosa from the sputum of cystic fibrosis patients were successfully transformed with plasmid pUC19 1.8. The applied voltage and the electroporation buffer composition were shown to have the greatest effect on transformation efficiency. Freezing the cells and prolonged storage at -70 degrees C did not significantly affect the transformation efficiency. The clinical isolates tested had lower transformation efficiencies than PAO1.