We compared activity, diving behavior and response to prey by Dermochelys coriacea and Chelonia mydas during their first 8–10 weeks of development. We reared juveniles in the laboratory and, at two-week intervals, released them in the ocean for a brief trial. Each turtle towed a device used to measure its dive profile. All turtles swam throughout their trials, but D. coriacea swam more slowly than C. mydas. Dermochelys coriacea dives had V-shaped profiles and older turtles made longer and deeper (up to 18 m) dives than younger turtles. Chelonia mydas dives were usually shallow (<6 m) and consisted of three (V, S, and U) profiles. Older C. mydas made dives that were longer but only slightly deeper than those of younger turtles. Dermochelys coriacea fed throughout the water column exclusively on gelatinous prey Aurelia, ctenophores, and unidentified gelatinous eggs. Chelonia mydas fed near the surface on floating Thalassia and Sargassum or at shallow depths on ctenophores and unidentified gelatinous eggs but ignored large jellyfish (Aurelia). Thus, early in development the two species overlap in foraging area and in diet. However as D. coriacea grow they dive deeper where prey assemblages probably differ from those in shallow water where C. mydas feed. These distinct behavioral trajectories probably cause the niches of D. coriacea and C. mydas to separate spatially very early in their development.