Two experiments (Exp) were carried out. The first one was the in vitro gas production experiment using 50 ml-syringes in a complete randomized design with 6 treatments and 3 replications. The treatments were 0, 1, 2, 3, 4 and 5 % of Cat fish oil (CFO) in the total substrate dry matter (DM). The substrate used in the experiment was Para grass. The second experiment was a 4 x 4 Latin square design with the growing sheep (19.7 ± 1.43 kg). The treatments were the supplementation levels of Cat fish oil of 0, 1, 2 and 3 % (DM basis) corresponding to the CFO0, CFO1, CFO2 and CFO3 treatment. Each experimental period was for two weeks including one week for dietary adaptation and another one for the sample collection. Methane (CH₄) production was measured over a 24 h period while the sheep were in respiration chambers...