Transgenic plants of lettuce cultivar (cv.) ‘South Bay’ were produced by using Agrobacterium tumefaciens vectors containing the β-glucuronidase (GUS) reporter gene and the NPT II gene for kanamycin resistance as a selectable marker. High frequency of transformation, based on kanamycin resistance and assays for GUS expression, was obtained with 24 to 72-h-old cotyledon explants cocultivated for 48 h with Agrobacterium tumefaciens. After the cocultivation period, the explants were placed in selection medium containing 50 or 100 mg l⁻¹ of kanamycin, 100 mg l⁻¹ cefotaxime and 500 mg l⁻¹ carbenicillin for 10 days. Surviving explants were transferred every 14 days on shoot elongation medium. Progenies of R₀ plants demonstrated linked monogenic segregation for kanamycin resistance and GUS activity.