Protoplasts isolated from suspension cultured cells of orange (Citrus sinensis Osb.) were treated with a bacterial plasmid DNA carrying a chimeric gene consisting of the nopaline synthase promoter, the aminoglycoside phosphotransferase II [APH(3′)II] structural gene from the bacterial transposon Tn 5 and a terminator region from cauliflower mosaic virus DNA. Colonies capable of proliferating in a medium containing kanamycin (25μg/ml) possessed APH(3′)II activity and an intact foreign gene. The transformation frequency was recorded to be in the order of 10-6.