Diets of yellow tail barb Mystacoleucus marginatus (Valenciennes, 1842) deposited in Princess Maha Chakri Sirindhorn Natural History Museum, Faculty of Science, Prince of Songkla University were studied by using stomach content analysis. The samples were collected from 4 tributary streams in Songkhla Lake Basin, namely Klong Pa Payom, Klong Nathom, Klong Tamod and Klong Rattaphume, between July 2005 and May 2006. The specimens were divided into 4 size classes according to total length (cm), class 1: 0-3.0; class 2: 3.1-6.0; class 3: 6.1-9.0 and class 4: 9.1-12.0. Preliminary analysis suggested that there was an ontogenetic variation in diets of M. marginatus. The specimens in class 1, class 2 and class 3 fed mainly on phytoplankton, zooplankton, plant materials and aquatic insects. While larger specimens in class 4 feed on aquatic insects, Asiatic Freshwater clam, Corbicula sp. and small Atyid shrimp, Caridina sp. Ontogenetic changes in diets composition related to the development of digestive system in fish.