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Diet composition, including seasonal and ontogenetic variations, was evaluated in juvenile gag, Mycteroperca microlepis (Goode and Bean, 1880), from a nearshore open marine seagrass bed on the north coast of the Yucatan Peninsula. Stomach contents from 322 juveniles (6.6-36.0 cm total length, TL) were analyzed using percentage frequency of occurrence (%F), percentage number (%N), percentage weight (%W), and a dietary index (Q = %N · %W). Young gag preyed heavily on caridean shrimps (Q = 791, %F = 38), fishes (Q = 327, %F = 13), penaeid shrimps (Q = 287, %F = 16), and unidentified decapods (Q = 114, %F = 26). Dominant prey in stomach contents shifted from caridean shrimps (Q = 1072, %F = 41) during the warm season to fishes (Q = 1392, %F = 21) and penaeid shrimps (Q = 1098, %F = 33) during the cold season. Size-dependent shifts in diet were also observed, particularly when organisms reached a size of ?17 cm TL. Dominant prey were caridean shrimps (Q = 1186, %F = 45) for gag ? 17 cm TL and fishes (Q = 2379, %F = 35) and penaeid shrimps (Q = 1005, %F = 27) for gag (more than) 17 cm tl. Diet composition and ontogenetic changes in juvenile gag diet were similar, independent of nursery habitat (seagrass or oyster shell bed inside or outside of an estuarine system). Gag is therefore best considered an estuarine opportunist, with nursery ground habitat (i.e., seagrass meadows) being the final factor controlling juvenile settlement and growth.