This review summarizes the scattered literature on the effects of toxicants on the external and internal innate immunity of fish. Insecticides, heavy metals and surfactants have been the most frequently examined toxicants, whereas dioxins, furans and polychlorinated biphenyls have been tested less frequently. Studies to date have been conducted at the levels of cells in vitro, of fish in the laboratory and microcosms, and also of fish in the field. Among innate immune parameters, phagocyte respiratory burst appears especially sensitive to toxicants. Toxicant-induced alterations in external mucous production have also been observed repeatedly. Field studies have occasionally examined changes to melano-macrophage centers, but the meaning of such changes is not clear. Advances in basic knowledge of fish innate immunity should lead to improvements in monitoring fish health and predicting the impact of toxicants on fish populations, which is a fundamental ecotoxicological goal.