Several differences are revealed in the segmentation and setation of the antennule, maxillule, maxilliped and fifth legs between three genera of the family Metridinidae, Metridia, Pleuromamma, and Gaussia. The phylogenetic relationships are cladistically analyzed on the basis of 10 morphological characters. The cladogram shows that Gaussia and Pleuromamma form a sister group, with Metridia which is the first branch to diverge within the family. The presence Of quadritheks on the male antennules of Metridia and Pleuromamma seems to be an important synapomorphy. The loss of quadritheks in Gaussia represents likely evidence of a character reversal into the ancestral condition.