A procedure has been developed which allows transformation of P. aeruginosa strain PAD with plasmid and bacteriophage DNA at 'a frequency of 10-6 per recipient cell. The method is similar in outline to. that developed for Escherichia coli. It involves growing the recipient cells to 3-5 × 108 per ml in nutrient broth, washing the cells with 0·1 M MgCl2, resuspending in 0 ·175 M CaCl2 for 20 min, exposing to DNA for 1 h and then heat pulsing at 42°C for 1 min. Some plasmid markers are expressed immediately, whereas others require time for phenotypic expression.