‘Candidatus Xenohaliotis californiensis’ (or Ca.Xc) is the aetiological agent of withering syndrome, a chronic wasting disease affecting most if not all North American species of abalone, and has been described as a Rickettsiales -like prokaryote. Genetic data regarding this species are limited to the 16S rRNA gene. The inability to grow it axenically has hindered its genetic and genomic characterization and, in consequence, a thorough analysis of its systematics. Here, we amplified and sequenced five genes (16S rRNA, 23S rRNA, ftsZ, virD4 and virB11) of Ca.Xc from infected abalone to analyse its phylogenetic position. Phylogenies from concatenated DNA and amino acid sequences with representative genera of most Rickettsiales unequivocally place Ca.Xc in the family Anaplasmataceae . Furthermore, the family has two reciprocally monophyletic lineages: one leading to ( Neorickettsia , Ca.Xc) and the other to (( Ehrlichia , Anaplasma ), Wolbachia )). A molecular-clock Bayesian reconstruction places Ca.Xc as the most basal lineage in Anaplasmataceae . These phylogenetic hypotheses shed light on patterns of host evolution and of ecological transitions. Specifically, Neorickettsia and Ca.Xc inhabit aquatic hosts whereas the remaining Anaplasmataceae are found in terrestrial hosts. Additionally, our evolutionary timeline places the directly transmitted marine Ca.Xc as the basal Anaplasmataceae , ancestral to both freshwater and terrestrial species with adaptations leading to more complex life cycles involving intermediate vectors or reservoir species; this supports the hypothesis of a marine origin for this bacterial family.