The sites of insertion for two lysogenic bacteriophages have been mapped on the chromosome of Clostridium perfringens strain CPN50 using two techniques based on pulsed field gel electrophoresis. Phage phi 29 was mapped to the 1 Mb region of the 3.6 Mb genome, near nanH which encodes a potential virulence factor, while phi 59 was found to have inserted at 2.9 Mb.