The Asian citrus psyllid, Diaphorina citri Kuwayama (Homoptera: Psyllidae), is an invasive pest that vectors citrus greening disease, which was recently detected in Florida. During the fall of 2005, deceased adult D. citri that were apparently killed by a fungal pathogen were collected at four sites in central Florida. Scanning electron microscopy revealed that the fungus had branched synnemata which supported monophiladic conidiogenous cells, a phenotype typical of Hirsutella species. In vitro cultures of the fungus were slow-growing on rice and produced synnemata six weeks post-inoculation. The fungus was maintained in vivo by exposing healthy D. citri from a laboratory colony to the synnemata borne on cadavers and rice cultures. Infected adult psyllids had altered behavior and septate hyphal bodies in their hemolymph. High levels of mortality were observed in adult and immature D. citri that were exposed to the fungal pathogen in laboratory bioassays. A high-fidelity polymerase chain reaction (PCR) assay was used to amplify the 28S rRNA and ß-tubulin genes of the fungus for identification. The morphological and phylogenetic data indicated that the fungus was a novel isolate related to Hirsutella citriformis Speare. Isolate-specific PCR primers were designed based on the unique putative intron region of the ß-tubulin gene and used to distinguish the psyllid pathogen from related Hirsutella isolates. Additional research is needed to evaluate the potential for using the fungal pathogen in an integrated pest management program to suppress D. citri populations in Florida.