ABSTRACT: We used stable isotopes and stomach content analyses to describe diet of red snapper Lutjanus campechanus in the northern Gulf of Mexico (GOM). Approximately 1000 fish were collected over 2 yr to test possible effects of ontogeny, habitat, and a non-trawl artificial reef permit area on red snapper diets. Stable isotopes of carbon (δ¹³C), nitrogen (δ¹⁵N), and sulfur (δ³⁴S) were measured in both red snapper and potential primary producers. Ontogenetic shifts in diet occurred with increasing red snapper size-at-age, resulting in higher δ¹³C and nitrogen δ¹⁵N and lower δ³⁴S in larger fish. Stomach content results supported ontogenetic shifts in diet by showing a change in diet from zooplankton, mysid shrimp, and squid for juvenile red snapper (ages 0 to 1) to diets dominated by benthic crustaceans and fishes for adults (ages 2+). Habitat-specific differences in isotopes and stomach contents of similarly sized fish were identified; however, feeding differences appeared to reflect ontogeny more than habitat type. In addition, seasonal differences, both in δ¹³C and in prey identified from stomach contents, were detected, but were minimal. Red snapper from areas outside a single non-trawl reef permit area had higher δ¹⁵N and lower δ³⁴S values than for conspecifics collected inside the non-trawl reef permit area. This study highlights the use of stable isotopes in detecting red snapper feeding differences inside and outside of an artificial reef permit area in the northern GOM, but additional studies are needed to verify if similar trends are present in other areas.