ABSTRACT Knowledge on fasting heat production (HEf) of fish is key to develop bioenergetics models thus improving feeding management of farmed species. The core of knowledge on HEf of farmed, neotropical fish is scarce. This study assessed the effect of body mass and water temperature on standard metabolism and fasting heat production of pacu, Piaractus mesopotamicus, an omnivore, Neotropical fresh water characin important for farming and fisheries industries all through South American continent. An automated, intermittent flow respirometry system was used to measure standard metabolic rate (SMR) of pacu (17 - 1,050 g) at five water temperatures: 19, 23, 26, 29 and 33 °C. Mass specific SMR increased with increasing water temperature but decreased as function of body mass. The allometric exponent for scaling HEf was 0.788, and lied in the range recorded for all studied warm-water fish. The recorded van't Hoff factor (Q10) for pacu (2.06) shows the species low response to temperature increases. The model HEf = 0.04643×W0.7882×T1.837 allows to predict HEf (kJ d-1) from body mass (W, kg) and water temperature (T, °C), and can be used in bioenergetical models for the species.