A strain of Aeromonas hydrophila producing copolyesters of 3-hydroxybutyrate and 3-hydroxyhexanoate, abbreviated as PHBHHx, was successfully transformed by electroporation. The plasmid used was a broad host range plasmid pBBR1MCS. Electroporation conditions were varied systemically to develop an electroporation protocol. The optimal yield of transformant was approximately 4£102 CFU/ g DNA at 12.5 kV/cm and 1000, resulting in a time constant of approximately 5ms. The A. hydrophila transformants expressed plasmid-encoded resistance to chloromphenicol. Plasmid DNA in the A. hydrophila transformant was stably maintained. This is the Wrst report of transformation of bacteria A. hydrophila.