Earlier papers (Bowden & Chadderton 1962, Chadderton & Montagu-Pollock 1963, Chadderton 1964) described the damage caused to various types of crystal by energetic fission fragments, and interpreted the electron microscope observations of damage on the basis of current theories of energy loss of charged particles, and of image contrast in the microscope. In this paper, electron microscope observations of fission fragment irradiated single crystals of lead iodide are described. This compound is heat-sensitive, and might therefore be expected to show damage through lattice excitation following the passage of a heavily ionizing energetic particle. The nature of the damage is sensitively dependent on the thickness of the crystal specimen, with tracks visible in thin crystals and a random array of dislocation loops in thicker ones. The variation in track visibility and appearance with thickness is described, and the possible mechanisms of energy loss which might account for these variations are discussed. The proximity of the crystal surfaces to the path of a fission fragment is a critical factor in determining the visible effects. Observations of damage in cadmium iodide are also discussed. The random distribution of dislocation loops in thick crystals, where surface effects are no longer influential, indicates a high mobility of the point defects created in the wake of a fission fragment by direct collision and thermal processes. Some possible mechanisms for the motion of energetic knock-ons in the lattice are suggested. These will be investigated more fully in part II.