The impact of the source of dietary fat and the level of dietary fiber on digestibility and energy metabolism were studied in human (six male, six female) volunteers. Subjects were divided into two diet treatment groups, high fiber [29.0 g total dietary fiber (TDF)/d] and low fiber (18.6 g TDF/d), for the duration of the study. Each participated in three, 2-wk controlled feeding periods. Either beef tallow (BT), corn oil (CO) or carbohydrate (CHO) was added (25% of diet energy) to a base diet in a three-way crossover study. Energy expenditure, substrate oxidation and digestibility determinations were conducted at the end of each period. The protein, fat and CHO digestibility of the base diet was significantly different between the fiber levels. The digestibility (high-fiber/low-fiber) averaged 82%, 90% for protein, 96% and 98% for fat. After adjusting for TDF, the CHO digestibility averaged 96% and was not different between fiber levels. The digestibility of the added CO and BT was 99.6 and 99.8% respectively, and was not significantly different between the fiber levels. No significant differences in 24-h energy expenditure existed nor the thermic effect of food due either to fiber level or between the CHO, BT or CO. Fat oxidation in subjects consuming the low-fiber diet was 14% higher (P < 0.03) with the BT treatment than with the CO treatment but not different in those that consumed the high-fiber diet. The energy value of the two fat sources was not different but their utilization by individuals near energy balance may lead to differences in long-term weight maintenance.