The study was performed to evaluate the use of gut weed (Enteromorpha sp.) as protein source to substitute soybean meal protein in the diets for postlarval tiger shrimp (Penaeus monodon). A diet without gut weed meal considered as a control was compared with four experimental diets that soybean meal protein was replaced by different gut weed protein levels, namely 15, 30, 45 and 60%. All diets were formulated to be equivalent in crude protein (40%) and lipid (7%). The feeding trial was conducted in 120 L plastic tanks filled with water at a salinity of 10 ppt, and provided continuous aeration. Thirty shrimp postlarvae with mean initial weight of 0.033g were stocked in each tank and fed the test diets for 45 days. At the end of experiment, survival of the shrimp was not affected by the feeding treatments, ranging from 84.4 to 88.9%. Overall, the growth rate and feed efficiency of the shrimps fed the gut weed-based diets were comparable to or better than animals fed the control diet. Moreover, the shrimps fed the diets containing gut weed meal showed a better formalin resistance than the ones fed the control diet, significant difference was only observed between the 30% replacement treatment and the control group.