In many developing countries, aquaculture expansion is limited not only by water scarcity, but mainly by the construction costs of fish ponds. This paper describes an experiment to avoid these limitations by using existing irrigation reservoirs for aquaculture production. The study involved immersing floating cages of tilapia fish in operational irrigation reservoirs in Israel and in irrigation channels in the Guanacaste province of Costa Rica. Fish yields in the reservoirs were high. In cages with 200 fish per cubic meter, the weight of the fish increased from 190 g to 216 kg in 36 days. In the irrigation channel, cages stocked with 300 small fish yielded an increase of 81 kg per cage over a 11-day period. Preliminary experiments were also conducted in Israel on the mullet fish species, which indicate that it is possible to rear this fish in a monoculture in channels. However, due to mullet's sensitivity to handling and jumping habits, it is convenient to start the culture with smaller fish. The mean increment of fish weighing 130 g each at a density of 11 fish per cubic meter was 240 g in a period of 100 days. In conclusion, though some problems arise in the practice of fish growth in cages, they can be solved. modified).