The structure of a 1.40 weight per cent carbon steel during the first stage of tempering (up to 200 C) was studied by transmission electron microscopy. Distributions consisting of either randomly arranged spherical or aligned platelike carbide particles were observed after tempering at 150 C. The origin of both of these types of distributions can be explained by heterogeneous nucleation on and growth of carbide particles along the dislocation networks of the tempered martensite.