Single stained DNA molecules can be visualized by fluorescence microscopy (1). Therefore, perhaps single events of DNA metabolism can also be visualized by fluorescence microscopy. Problems to solve include (a) photobleaching of stained DNA, (b) photolytic damage to macromolecules necessary for metabolism, including DNA, (c) inhibition of metabolism by the process of preparation of the specimen, and (d) interference with image formation by macromolecules that either must be or by chance are present during metabolism. These problems have been solved, at least in part, during a study of the in vitro packaging of bacteriophage T7 (2). Packaging occurs in T7 capsids that had previously been assembled without assistance of DNA. The DNA substrate for packaging is a head-to-tail multimer (concatemer) of the terminally repetitious, nonpermuted T7 DNA. The metabolic event observed in ref. 2 was capsid-dependent specific cleavage of concatemers during packaging. The concatemers had formed a DNA network before cleavage.