Of 11 possible hybrids of species in the family Esocidae, six were known from nature and they are reviewed. The occurrence of a seventh, Esox lucius × Esox americanus americanus, is recorded. Of 22 possible reciprocal combinations of these species, five were previously known, five are still unknown, and 12 are newly described here. These 12 are based on artificial hybrids of known gametic constitution. Data are given on: description (young and oldest material available); growth; fertility; vitality and meristics are compared with parent populations. The low level of interspecific sterility was surprising. Some interspecific sterility exists between the two largest (Esox masquinongy Mitchill, Esox lucius Linnaeus) and the two smallest (Esox americanus Gmelin) forms. Total sterility does not exist as artificially one or other of the reciprocals was successful in each cross, including Esox masquinongy × Esox americanus. The hybrids exhibited the blending and intermediate nature usual in fish hybrids but colour pattern seemed tied to a particular parent. In nature only certain species hybridize, but it would appear that incompatibility of gametes has proceeded only to a limited extent. Other factors such as distribution, habitat, size, and behavior may be preventing hybridization.