We describe a technique wherein single muscle fibers of intact mice are made transgenic by intracellular injection of DNA expression vectors for the reporter genes lacZ and green fluorescent protein (GFP). Application of in vivo imaging techniques allowed identification of single cells during the injections, and of the same single cells when the muscle was reexposed days or weeks later. DNA concentration by itself had little effect on fiber survival or expression efficacy, but it seemed crucial to exceed a threshold of about 10 6 injected plasmid molecules in order to obtain expression. On the other hand, experiments with coinjection of two different plasmids suggested that relatively few individual molecules were eventually transcribed in expressing cells. Plasmid DNA remained localized to the injection site, and expression was confined to nuclei in the vicinity. Expression was stable, as reporter was detected 2-61 days after injection.