A fish species can be called mesopelagic if it spends the day in the mesopelagic zone. The mesopelagic zone has been defined in different ways based on depth, temperature and light regimes. For the present purpose, depth seems to be the best criterion and mesopelagic fish can thus be defined as species generally spending the day at depths between approximately 200 and 1 000 m. Generally they perform a diel migration, coming to the upper 200 m or even to the surface during the night. The present report briefly reviews the systematics ecology and life history of important groups of mesopelagic fish. Further, some methods used for abundance estimation of these fish are described and discussed. The bulk of the report reviews the present knowledge of abundance and species composition of mesopelagic fish in each of the FAO statistical areas. Although the data in most cases are few, an attempt has been made to give tentative estimates of the biomass and production in the various areas. When available, information on the life history of the most important species in each area is also given.