Third instars of the diving beetle Notaticus fasciatus Zimmermann (Coleoptera: Dytiscidae) are described for the first time, representing the first description of larvae from Aubehydrini, the only remaining tribe of Dytiscinae unknown as larvae. The description is based on larvae suspected to be N. fasciatus collected in French Guiana. The specimens were associated with adults by analyzing an 806-bp fragment of cytochrome c oxidase I. Adults of N. fasciatus and 19 other species were included representing all tribes and most species of Dytiscinae found in northern South America with multiple representatives of some for a total of 33 terminals. Data were analyzed using parsimony and p-distances. The parsimony analysis resulted in a single tree. The unknown larva grouped with adult specimens of N. fasciatus (bootstrap 100%). Distance between them averaged 0.01 (0.01–0.02) compared with 0.02 (0–0.06) between other conspecifics and 0.13 (0.05–0.17) between different species. N. fasciatus larvae differ from other Dytiscinae in the following characters: 1) head capsule subtriangular; 2) occipital suture absent; 3) anterior margin of frontoclypeus convex; 4) dorsal margin of foramen magnum straight; 5) dorsal pair of stemmata unmodified; 6) antennomere I, maxillary palpomere I, and labial palpomeres I and II not subdivided; 7) maxillary palpomere II subdivided; 8) ligula absent; 9) galea well developed; 10) natatory setae present on dorsal margin of coxae, absent along ventral margin of trochanters and tarsi; 11) tarsal claws not serrate; 12) abdominal tergites fully developed; and 13) urogomphus well developed, shorter than last abdominal segment, and without natatory setae.