Two international research cruises on board of the research vessel 'Ichthys' the first one in Aug-Sept 1971, and the second other in June-July 1972, have shown the reproduction of Thunnus thynnus, T.alalunga, Auxis thazard and Sarda in the surroundings of the Balearic Islands. The observations were made on the planctonic stages of these spp and on the physico-chemical and biological conditions of the environment. Some data were also collected on the presence of juvenile or mature stages of these fishes and on their fishery.