Studies on relative energy budget in different weight groups in a fresh water siluroid fish, Mystus cavasius (Ham.) has been made at water temperature 30.0±1.0°C. The total energy available in feed (C)/day/fish ranged from 2192.4 Joules (in 12.2g fish) to a value of 26989.2J (in 102.0g fish). The percent energy value ranged from 12.3-13.6, 7 (fixed), 35.2-36.8, 36.0-43.3 and 0.6-8.2% respectively in faeces (F), Urine formation (U), growth (P), respiratory metabolism (R) and miscellaneous activities (M.A.). The details have been discussed in this paper.