The amount of fat in the diet is known to influence body weight and body composition, but it is not clear whether dietary fat composition can affect body composition independently. We studied the effects of six months of feeding diets containing lard (L), corn oil (CO), fish oil (FO) or medium chain triglycerides (MCT) on body weight and body composition in adult male Wistar rats. Although FO fats ate slightly less total energy than the other groups, there were no differences among groups in body weight at any time during the study. However, body composition, the composition of depot triglyceride, body fat distribution and insulin resistance were all influenced by the type of fat in the diet. FO rats had less total body fat, less intra-abdominal fat, and less insulin resistance than all other groups. Although some of these metabolic effects may have been secondary to a slightly lower energy intake, we believe these data demonstrate the potential impact which dietary fat composition can have on metabolism and body weight regulation.