The ostracode genus Poseidonamicus has been widespread and abundant in deep-sea sediments since the Eocene. Despite its prominent role in a number of evolutionary studies, species identification in this genus is often difficult and phylogenetic relationships among its species are not well understood. Here I present the findings from a comprehensive study of this genus with the purpose of discovering novel phylogenetic characters and clarifying species relationships. I briefly describe the adult carapace and trace some of the major morphological changes that occur over the last several instars. I focus particular attention on the arrangement of fossae in the reticulate mesh; these features have been shown in other ostracodes to correspond to underlying epidermal cells. I describe the development of fossae in the region posterior to the adductor muscle scars, and hypothesize a sequence of specific cell divisions to account for the addition of fossae over ontogeny. Phylogenetic characters were derived from many different types of characters, including aspects of carapace shape, the presence and location of pores, characteristics of specific ridges and spines, and the relative position of homologous fossae in the reticulum. A parsimony analysis of 42 characters and 40 operational taxonomic units (36 ingroup and four outgroup) resulted in a set of optimal trees whose strict consensus is relatively well resolved, well supported, and generally consistent with the order in which taxa appear in the fossil record. The monophyly of Poseidonamicus is supported, as is the monophyly of all deep-sea members of this genus. Within the clade of deep-sea Poseidonamicus, several subgroups are recovered with varying levels of character support. In addition to providing a general framework for understanding morphological evolution in this genus, the results of this phylogenetic analysis have two specific implications for the evolution of sightedness in this genus. First, because Poseidonamicus ocularis is nested deeply within a clade of entirely deep-sea species, its putatively ocular features are probably not related to vision. Second, there has likely been just a single transition from sighted to blind in Poseidonamicus, coincident with its colonization of the deep sea. No support is found for the recent suggestion that sighted shallow-water dwelling Poseidonamicus species may have evolved from blind deep-sea ancestors, although data from additional taxa will be necessary to test this hypothesis more fully.