This paper deals with an experiment carried out on the shoaling reaction in fish for the purpose of giving a qualitative and quantitative expression to the shoaling behaviour. In five species living in shoals as shown in Table 1 the reaction of an individual fish on another one or several individuals took in a glass vessel was investigated, and the bearing of the results obtained was discussed, especially on the relation between the reaction (P) and number of individuals in the vessel (N) which was represented by an empirical formula, P=K•log N+C, within a certain limit of N.
