An analysis of larval cranial architecture and mandibular geometry supports a hypothesis that larvae of Dytiscus carolinus Aubé, 1838 preferentially feed on active immature vertebrates and invertebrates. Encased caddisfly larvae are probably only coincidental components of its prey regime. Ontogenetic changes in cranial and mandibular morphology are described and interpreted as indications that first instars exploit in situ prey regimes with characteristics different from those exploited during the third and possibly second stadium. Pronounced developmental changes in temporal curvature are indicative of increases in the mass and volume of adductor muscles but without corresponding changes in gear ratios. Intermandibular distances increased at a constant rate, but significant differences between intermandibular distance and head length and width ratios occurred between first and second instars and first and third instars. Significant variation in mandibular morphology and geometry during larval development included changes in proportional mandibular length, midpoint depth, angles of attack, and lateral and medial arcs.