The roundscads (Decapterus spp.) form the most important fishery in the Philippines today. More than 30% of the total fish production of about one million tons reported in 1970 was composed of the two species of roundscads: both species are caught in commercial quantities throughout the Philippine archipelago. The fishery is based on the capture of the immature forms, since adults are not available. The absence of adults may be due to the loss of phototaxic reponse of adults to the lights used to lure the fish into the gear (the bagnet), or the adults may have moved out of the fishing grounds. The biology of these species is reviewed and a general introduction of the geography and oceanographic background of the Philippine waters provided. A description of the oceanographic conditions of the Sulu Sea, which is one of the most important fishing grounds for roundscads, was made. Through a brief discussion of the zooplankton, egg and larval distribution of tuna and tuna-like fishes, as well as other fish species, an assumption of the probable spawning areas of roundscads was advanced. For the pelagic fish, the bagnet and purse seine fisheries were shown to be the most productive; the landing from these methods exceeded 50% of the total fish production is explained by the increased efficiency attained by the bagnet and purse seine, resulting from the use of brighter lights, better synthetic (kuralon) nets, larger vessels, and more efficient gear.