The distribution of the branchiobdellidan Holtodrilus truncatus (Liang, 1963), which is symbiotic on atyid shrimps, was surveyed in 26 river catchments, covering almost all of the Kii Peninsula, western Japan. Holtodrilus truncatus has been recorded in 10 rivers located from the southwest to northeast regions of the peninsula. Paratya compressa was the dominant host species in all rivers, with a lower prevalence on Caridina leucosticta, C. multidentata, C. typus, and Neocaridina denticulata. The longitudinal distribution of H. truncatus was limited to the middle and upper reaches, while its main host P. compressa occurred widely from the lower to the upper reaches. The numbers of H. truncatus and their cocoons per host increased in summer and decreased in winter, in accordance with the abundance of the host P. compressa. The body size of P. compressa was larger in individuals with H. truncatus than those without H. truncatus in some rivers, while in another river, sizes of the two groups were not different. A salinity tolerance experiment revealed that the threshold of salinity tolerance of H. truncatus occurred between 0-0.5%. A choice experiment between two host species showed that H. truncatus preferred C. leucosticta to C. multidentata, P. compressa to C. multidentata, and P. compressa to C. leucosticta.