Histidine (His)-to-Aspartate (Asp) phosphorelay signal transduction systems are generally made up of a "sensor histidine (His)-kinase", a "response regulator", and a "histidine-containing phosphotransmitter (HPt)". In the higher plant, Arabidopsis thaliana, results from recent intensive studies suggested that the His-to-Asp phosphorelay mechanism is at least partly responsible for propagation of environmental stimuli, such as phytohormones (e.g. ethylene and cytokinin). Here we compiled the members of the HPt family of phosphotransmitters in Arabidopsis thaliana (AHP-series, Arabidopsis HPt phosphotransmitters), based on both database and experimental analyses, in order to provide a comprehensive basis at the molecular level for understanding the function of the AHP phosphotransmitters that are implicated in the His-to-Asp phosphorelay of higher plants.