Direct observation of the effects of fission events has been made in thin films of Al and Pt by transmission electron microscopy.A very thin layer (10 to 20 A thick) of UO2was incorporated between evaporated metal films. The samples were exposed to a total thermal neutron flux of 2.8×1015n/cm2. The fission fragments produced tracks in the matrix, the characteristic appearance of which is strongly influenced by the thickness and the atomic number of the matrix.The mechanism of track formation is discussed from the viewpoint of dissipation of energy by the thermal spike mechanism.