In an earlier paper [1], a detailed examination was made of the structure of a small portion of the genetic map of phage T4, the rII region. This region, which controls the ability of the phage to grow in Escherichia coli strain K, consists of two adjacent cistrons, or functional units. Various rII mutants, unable to grow in strain K, have mutations affecting various parts of either or both of these cistrons. The topology of the region; i.e., the manner in which its parts are interconnected, was intensively tested and it was found that the active structure can be described as a string of subelements, a mutation constituting an alteration of a point or segment of the linear array. This paper is a sequel in which inquiry is made into the topography of the structure, i.e., local differences in the properties of its parts. Specifically, are all the subelements equally mutable? If so, mutations should occur at random throughout the structure and the topography would be trivial. On the other hand, sites or regions of unusually high or low mutability would be interesting topographic features.