AbstractThe freshwater sardineClupeichthys aesarnensisis a zooplanktivore like many clupeids. In Ubolratana reservoir, Thailand, it feeds selectively on larger zooplankters, especially Cladocera and to a lesser extent on Copepoda. Other zooplankters are also consumed together with both living and dead insects and occasionally its own young. It feeds during the day. Our results indicate that the slight seasonal differences in food consumed probably reflects differences in availability at different seasons and sites. The fish grows in weight, as expected, at a higher rate than the cube of its length. The condition factor is the same for males and females and shows some seasonal variation. Data on fecundity and gonadosomatic index mark this fish as highly fecund and a year‐round spawner. Its short life span, favourable growth and high fecundity make it a valuable source of cheap protein. In 1990, 17% (about 300 tons) of the fish catch from Ubolratana came from this fis