An experiment was carried out in a household in Hoa Loi village, Chau Thanh district of Tra Vinh province to evaluate the effects of replacing dietary fish meal by catfish (Pangasius hypophthalmus) by-product on growth performance and carcass composition of crossbred Muscovy ducks. The ducks (n=150) at four weeks of age were arranged in a completely randomized design with 5 treatments and 3 replicates with 10 birds (balanced for sex) per experimental unit. The treatments were diets in which fish meal protein was replaced by catfish by-product protein at levels of 0, 25, 50, 75% and 100%. The  rest of the diet was maize grain and rice bran. Feed intake and live weight gain followed a curvilinear pattern  increasing to maximum values with 75% replacement of fish meal by catfish byproduct protein then decreasing. There was a 50% increase in heart weight of the ducks when the catfish by product replaced all the  fish meal (40% of the diet DM).  It is concluded that catfish byproduct meal can replace up to 75% of the fish meal protein with slight improvement in growth rate and no effect on carcass quality. However, complete replacement of the fish meal by catfish byproduct  may not be advisable in view of potential negative effects on health status as evidenced by a major incease in heart weight of the ducks at this level.