The work reported here is a study of the peritoneal fat of the Amazonian fresh-water fish, Colossoma macropomum, known locally as "tambaqui", with respect to its physical and chemical characteristics and its potential use as a cooking fat. Samples were taken at different times, June and October, 1977 and February and April, 1973 Analyses revealed that the fat of the tambaqui is more similar to animal and vegetable fats, them to the oils of saltwater fish. Stability tests were performed on the deodorized fat by following the rate of increase of rancidity by measuring the peroxide and the TBA values. It was necessary to add antioxidants to prevent the rapid oxidation of the fat. Potential of Colossoma fats in cooking were tested by deep-frying potatoes in, (1) deodorized fish fat and, (2) commercial soybean oil. In the two sensory tests made on the first day after deodorization and 94 days later, only 33% of the panel prefered the potatoes fried in the soyben oil, and 60% were indifferent to the fat used for frying.