The present study was conducted to evaluate the potential replacement of fishmeal protein by gut weed (Enteromorpha sp.) protein in practical diets for mudskipper (Pseudapocryptes elongatus) fingerlings. A control diet containing fishmeal as main protein source was compared with five experimental diets in which fishmeal protein was replaced by increasing dietary levels of gut weed protein, namely 10 percent, 20 percent, 30 percent, 40 percent and 50 percent. All diets were fonnulated to be equivalent in crude protein (30 percent) and lipid (7 percent). After 60 days of feeding trial, survival of experimental fish was not affected by the feeding treatments ranging from 81.1 to 88.9 percent. When comparing the control diet with the treatments of 10 percent, 20 percent, 30 percent and 40 percent gut weed protein replacement, no significant differences (P0.05) were found for growth perfonnances and feed efficiency. Particularly, fish group received the 10 percent gut weed protein substitution showed slightly higher growth than those in the control treatment The proximate composition (water content, protein, ash and phosphorus) of fish carcass in all treatments was similar except the lipid and calcium levels in the treatments of 40 and 50 percent replacement were significantly lower compared to the control treatment (P0.05). These results indicated that gut weed protein could replace up to level of 40 percent of fishmeal protein in practical diets for mudskipper fingerlings.