The metabolizable energy of two diets differing in fat and fiber content was measured in balance experiments in 42 adult male subjects. Subjects received each diet for 10 wk in a crossover design. Average fiber consumption from mixed sources (cereals, legumes, fruits and vegetables) while consuming the low fiber, high fat diet was 34 g/d and 64 g/d while consuming the high fiber, low fat diet. The percentages of energy from protein, fat and carbohydrate were 16, 18.5 and 65.5, respectively in the high fiber, low fat diet and 14.5, 36 and 49.5, respectively, in the low fiber, high fat diet. Seven-day composite collections of food, urine and feces were made during the 10th wk of the study. The freeze-dried food and fecal samples were analyzed for total energy content and for protein, fat, moisture, ash and carbohydrate. Urine samples were analyzed for nitrogen and energy content. The digestibility of the energy-containing nutrients was significantly lower when subjects were consuming high fiber, low fat vs. low fiber, high fat diets (95.4 vs. 97.0, carbohydrate; 92.5 vs. 95.5, fat; 83.7 vs. 89.4, protein; and 91.4 vs. 94.3, energy.