The survival of a genetically engineered strain of Pseudomonas fluorescens introduced to the seeds, stems and leaves of maize plants was monitored using viable cell counts on selective agars. The bacterium was introduced to the seeds under vacuum and stored at either room temperature or 5°C. The bacterium was detected after 1 d, but not after 8 d, in seeds stored at room temperature, whereas at the lower temperature it was still detected after 43 d. Other inoculated seeds were grown either in a nutrient solution or in potting compost. Studies of survival within both seed remnants and roots, and on the aerial parts of the plants, demonstrated that the majority of bacteria remained in the seeds with some movement into the roots. Seeds retained a greater colonization frequency than roots in plants grown in both soil and nutrient solution. Those plants grown in nutrient solution maintained higher bacterial populations in seed remnants and roots than those grown in soil. Viable counts of P. fluorescens within seed remnants and roots decreased in soil-grown plants over a 43 d monitoring period. No introduced organisms were detected in either stem core sections or other aerial parts of any of the inoculated plants, yet the cores were shown to contain an indigenous population of up to 4.8 × 104 cells g−1 tissue. When mature plants were injected with the bacterium viable counts of the pseudomonad below and above the injection points demonstrated the ability of the bacterium to translocate in the stem after the wounding process. P. fluorescens applied to the leaf surfaces remained viable for at least 20 d but showed great variations in numbers with values ranging from 0 to 1.4 × 106 g−1 tissue. The practical applications of introducing genetically engineered microorganisms into plants as control measures against pathogens and insect pests are briefly discussed.