The isopod Ceratothoa verrucosa (formerly as Rhexana verrucosa or Rhexanella verrucosa) has been known for a long time as a parasite of red seabream Pagrus major in Japan (see Yamauchi et al., 2004). However, its biological and ecological aspects have been studied only fragmentally except for descriptions of its life cycle (Hiraiwa, 1934;Sanada, 1941).During surveys on the stock assessment of red seabream in Iyo-Nada, part of Seto Inland Sea (Fig. 1), by Ehime Prefectural Fisheries Experimental Station (now Fisheries Research Center, Ehime Research Institute of Agriculture, Forestry and Fisheries) in early 2000s, C. verrucosa infection was frequently noticed. Since red seabream is one of the most important fish species for Ehime Prefecture, effects of the isopod infection on the red seabream stock in the Iyo-Nada area were needed to be estimated. We initiated a long-term study on the biology and ecology of the isopod, including its infection cycle, growth and life span as well as on the effects of infection on the survival of red seabream. Materials and Methods Fish samples and isopod infectionRed seabream, Pagrus major, caught commercially by mid-water trawl in Iyo-Nada, were purchased monthly at a local fish market from January 2003 to December 2007. The number of fish surveyed at each sampling varied from 20 to 201; no or few numbers of fish were available in some winter months due to no commercial fishing. Fish, either fresh or frozen, were examined, comprising 4,623 individuals from 145 mm in fork length (0+ year-old) to 576 mm (14-year-old) (Table 1). They were sexed and their age was determined by the number of scale rings (Takaba, 1997). Smaller fish were