Technical Abstract: Candidatus Liberibacter asiaticus (Las) bacterium has been strongly associated with huanglongbing (HLB), or citrus greening, which is currently the most devastating citrus disease worldwide. HLB is transmitted by the Asian citrus psyllid (ACP), Diaphorina citri in a persistent manner but its interactions with the psyllid vector particularly at the organ and cellular levels are poorly understood. Here, we tested several fluorescent in situ hybridization (FISH) protocols for the localization of Las in hemolymph smears and dissected organs of ACP adults that fed on HLB-infected citrus trees in the field or laboratory, and in sections from HLB-infected citrus leaves. Las was detected by FISH in the filter chamber, midgut, Malpighian tubules, hemolymph, salivary glands, ovaries and in muscle and fat tissues of HLB-infected ACP, as well as in the phloem of infected citrus leaves. Las appeared as pleiomorphic bodies or short thin rods that were much more spatially dispersed and individually distinct in citrus leaf phloem and in ACP hemolymph, but more densely aggregated in cells of the salivary glands and other ACP organs/tissues. FISH results indicated that the proportion of Las-infected salivary glands was significantly lower than that of infected alimentary canals or other organs. Our results show the near systemic infection of ACP by Las and confirm previous Polymerase Chain Reaction (PCR) results suggesting that the salivary glands constitute an important barrier to Las infection and/or transmission by ACP.