This review deals essentially with the influence of environmental temperature on the immune response of fish and on their immune resistance to pathogens. The first part of this paper (Sections II and III) is concerned with the resistance of fish to infections and the influence of different natural or experimental stresses, including the influence of the environmental temperature, on both the humoral immune response and the phagocytic apparatus. On the basis of the reported data, as well as field observations, an attempt was made to explain the mechanism of infection in fish. It was stated that microbial contamination, environmental stresses, temperature, and specific immunity are the major determining factors in the infection of fish. Specific immunity may have a palliative action enabling the fish to surmount infection despite the stress; however, it could be reduced or abolished at low temperatures. The second part of this review deals with temperature control of experimentally induced humoral immune response in fish. The primary and secondary responses, and immunologic memory, as well as the carrier effect and specific tolerance phenomena and their regulation by means of temperature modulations, are described. The mechanism by which the ambient temperature affects the immune response of fish and the determination of the nature of the temperature‐sensitive event as to its possible relationship to helper‐cell maturation and its temporal localization in the immune process are discussed in light of recent findings on cell cooperation in mammalians.