Behaviour of the Japanese striped knifejaw Oplegnathus fasciatus was studied by underwater observation and tagging experiment at Tajima coast in Japan Sea. The results obtained are as follows ; 1) The fish schools were small on the rocky reef, but were large on the artificial seaweed farm plant. 2) High rates of schooling were shown in juvenile and young fish respectively, both on the artificial seaweed fam plant and on the rocky reef. The rates of schooling on the artificial seaweed fam plant were higher compared with those on the rocky reef. 3) Approximately 92 % of tagged fish were recaptured within l0 km from the released point. 4) It was suggested that small schools and solitary individuals on the rocky reef were sedentary and large schools were migratory. It was also suggested that large schools on the artificial seaweed farm plant were staying there temporarily. 5) An unfavourable environment as sandy bottom seemed to force the fish to aggregate on the structure like the artificial \SeaWeed farm plant and the artificial reef. On the other hand, the large schools broke into smaller ones on a favourable environment as the rocky reef and the fish stayed there for a long period of time.
