Molecular systematics of the 'Navanax aenigmaticus' species complex (Mollusca, Cephalaspidea): Coming full circle. -Zoologica Scripta, 41, 374-385. Molecular evidence from the mitochondrial COI and 16S genes and the nuclear H3 gene indicate that the traditionally recognized cephalaspidean sea slug species Navanax aenigmaticus consists of three deeply divergent lineages with disjunct ranges in the eastern Pacific, western Atlantic and eastern Atlantic. Each of these allopatric lineages is highly variable in colour and body size, which hampers identification of some possible consistent differences between them. Some conchological differences between the three lineages seem to be correlated with the groupings resulting from the analyses of molecular data, but the results of the morphological studies are inconclusive. Because of the presence of well-supported divergences and molecular synapomorphies, these lineages are herein considered to be three separate cryptic species. A review of the literature and available type material was conducted to determine the valid name for each of the three species. In other to promote nomenclatural stability, the oldest name with a description that allows a positive identification was selected over older, taxonomically ambiguous names. The conclusion of this revision is that the valid names for the species are Navanax nyanyanus (Edmunds 1968) - eastern Atlantic, Navanax gemmatus (Mörch 1863) - western Atlantic and Navanax aenigmaticus (Bergh 1893) - eastern Pacific.