Electroporation proved to be an efficient transformation method for Citrus including mandarins which had not been transformed by the Agrobacterium method. Protoplasts were prepared from embryogenic callus of 'Ohta' ponkan (Citrus reticulata Blanco) and electroporation with exponential decay pulses was carried out in the solution containing the β-glucuronidase (GUS) chimeric gene coupled to the CaMV 35S promoter (pBI221). At 24 hr after incubation, significant GUS activity was detected in the cells by fluorometric assay. To optimize the transformation efficiency the parameter of electroporation including the capacitance, the voltage, CaCl₂ and DNA concentrations in the medium, and the heat shock to protoplasts were investigated. The transformation signals by the histochemical assay were also detected in newly formed callus colonies after 2 month culture of protoplasts.