Transgenic papaya plants were regenerated from hypocotyls and immature zygotic embryo after cocultivation with Agrobacterium tumefaciens LBA-4404 carrying a binary plasmid vector system containing neomycin phosphotransferase (nptII) gene as the selectable marker and -glucuronidase (GUS) as the reporter gene. The explants were co-cultivated with Agrobacterium tumefaciens on regeneration medium containing 500？mg/L carbenicillin？+？200？mg/L cefotaxime for one week. The cocultivated explants were transferred into the final selection medium containing 500？mg/L carbenicillin？+？200？mg/L cefotaxime？+？50？mg/L kanamycin for callus induction as well as plant regeneration. The callus derived from the hypocotyls of Carica papaya cv. Shahi showed the highest positive GUS activities compared to Carica papaya cv. Ranchi. The transformed callus grew vigorously and formed embryos followed by transgenic plantlets successfully. The result of this study showed that the hypocotyls of C. papaya cv. Shahi and C. papaya cv. Ranchi are better explants for genetic transformation compared to immature embryos. The transformed C. papaya cv. Shahi also showed the maximum number of plant regeneration compared to that of C. papaya cv. Ranchi. 1. Introduction Papaya (Carica papaya L.) is a tropical fruit having commercial importance because of its high nutritive and medicinal value [1]. It is also a good source of vitamins A and C and the proteolytic enzyme papain and chymopapain [2, 3]. The origins of papaya are south Mexico and Costa Rica [4]. Major papaya cultivating countries are India, Brazil, Mexico, Nigeria, Indonesia, China, Peru, Thailand, and Philippines [5]. Papaya cultivation has been affected by a number of diseases especially those caused by the papaya ringspot virus (PRSV) [6, 7], papaya bunchy top bacterium [8], and the fungus Phytophthora palmivora [9]. Traditional breeding of papaya cultivars for resistance to these maladies had limited success. The rapid development in biotechnology, especially genetic transformation of plants, has made it possible to introduce selected genes into plants to control plant diseases and pests. Crop improvement to solve disease problems of a tree species like papaya has been enhanced by gene transfer techniques [10]. Genetic engineering of plants has evoked great interest in developing modern technology for crop improvement. Many transgenic plants have been successfully produced with remarkable results such as resistance to chemicals, pests, and diseases [11]. The success of any genetic transformation protocol depends on three steps such
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