To advance the understanding of the diet of the greater lizardfish,Saurida tumbil(Bloch,1795) and its monthly and ontogenetic variations,an annual research was conducted from October 2008 to September 2009.Stomachs of 749 greater lizardfish,ranging in size from 124 mm to 375 mm in standard length(SL),were sampled monthly in the Beibu Gulf of the South China Sea,and analysis of their stomach contents showed Decapterus maruadsi to be the primary prey components of the diet,averaging 9.7%by number,55.5%by weight and 34.9%by percentage of Index of Relative Importance(IRI).Small benthic fish,Bregmaceros rarisquamosus and pelagic fish, Thryssa setirostris and Leiognathus riviulatus,with IRI percentages of 26.6%,4.3% and 2.5%,respectively were also relatively essential in the diet of this fish,whereas the other species of prey were only present in comparatively lower percentages.The research findings revealed that the mean repletion index and mean maturity coefficient varied considerably by month,while trophic diversity,calculated by Brillouin index H_z, proved to be relatively consistent throughout the year. Mean repletion index(RI) was highest(RI=7.61%) at the stage of Ⅵ-Ⅱ indicating that this group of greater lizardfish that reverted from Stage Ⅵ to StageⅡ increased their food input to compensate for energy loss during reproduction.On the other hand,the proportion of empty stomachs reached its peak with 61.5% while mean RI reached the second highest position with 5.98% at Stage Ⅴ,indicating that during the reproduction periods from March to April,and in June and November,greater lizardfish ceased feeding until digestion of the prior meal was completed or near completion.The 12 SL classes demonstrated significant differences with respect to prey weight but showed relative consistency in prey numbers.The tendency of mean prey weight increased the body size following with the improvement of the feeding organs and swimming ability of greater lizardfish.Using PRIMER 5.2 software,the cluster analysis,based on IRI values for the above SL classes,showed that the feeding habits of greater lizardfish were generally homogeneous among different sizes,particularly among the 136-255 mm SL classes,with similarities of more than 60%.In conclusion,the diet of greater lizardfish,Saurida tumbil varied significantly by month and ontogenetic development.These findings may assist in the development and improvement of marine ecosystem modeling for the Beibu Gulf,South China Sea.