Recently, frequent blooms of the commercially harvested rhizostome jellyfish Rhopilema esculentum have occurred in the Ariake Sea, western Japan. We attempted to estimate the abundance and biomass of the jellyfish in the inner and middle parts of the sea in the warm seasons of 2012. The maximum abundances of individuals occurring in the surface waters of these areas were around 408–525 （inner part, August and September） and 365（middle part, October） indiv. per km2, respectively. The jellyfish seemed to be distributed in patches. Based on the abundances in the inner and middle parts of the sea, the biomasses were estimated as approximately 3,264–4,200 t and 7,300 t in wet weight, respectively.
