The objective of this study was to compare growth performance and feed conversion ratios of rainbow trout (Oncorhynchus mykiss) and brook trout (Salvelinus fontinalis) juveniles in monoculture and duo-culture in freshwater and seawater under aquarium conditions. The fish were about 2-months old hatchery-reared brook and rainbow trout with initial weights of 0.934 +/- 0.033 (n= 360) and 1.014 +/- 0.019 (n= 360) g, respectively. The juveniles were kept in 10 L aquaria. Each aquarium contained 40 fish: 40 fish from each species for monoculture, 20+ 20 (rainbow trout+ brook trout) fish from each species for duo-culture. The fish were in 3 groups with three replicates in freshwater and seawater and 18 aquaria were used. At the end of the 45 day study; mean live weights of brook trout were 2.735 +/- 0.153 g, rainbow trout 2.925 +/- 0.262 g in monoculture and brook trout 2.354 +/- 0.186 g, rainbow trout 3.882 +/- 0.494 g in duoculture in seawater, brook trout 3.088 +/- 0.085, rainbow trout 3.364 +/- 0.093 g in monoculture and brook trout 2.164 +/- 0.169, rainbow trout 3.948 +/- 0.124 g in duo-culture in fresh water. While there were similarities between brook trout and rainbow trout in monoculture groups in sea water and fresh water, some differences were realized in duo-culture (p< 0.001).