So far the availability of seeds is still from the natural and seasonal. Therefore, the provision of seeds should be continuously developed. However, the problem is the mortality rate of fish larvae is very high. This is due to the period of the larvae is a critical period, which is among others related to the ability of larvae to take the feed and feed suitability with the size of the mouth opening. From this case, it is necessary to do research to know the size of the mouth opening of the larvae and the type of plankton consumed. So it is expected that the mortality of the seeds is low and is always available at all times. Method of climbing perch larvae sampling in Bangkau Lake waters is collecting climbing perch larvae sampled as many as 10 fish, taken 3 days after the eggs hatch, and then 3 days interval for 31 days. The results of this study were climbing perch larvae 3 days old the mouth opening size of 103.110 ± 1.709 μm (mean ± SD) until the climbing perch larvae were 23 days mouth opening size of 162.463 ±4.001 μm (mean ± SD), an increase in the size of the mouth opening is still small. But after climbing perch larvae were 23 days to 31 days old mouth opening size of 1019.15± 6.106 μm (mean ± SD), an increase in the size of a very large mouth opening.Based on the percentage frequency of the presence of plankton in the digestive tract, the predominant type of plankton eaten by climbing perch larvaenamely:Coconeis sp, Mougeotia sp,Brachionus sp, Diatoma sp,Navicula sp dan Keratella sp.Plankton waters as a natural food that is consumed by climbing perch larvae has made changes to the size of his mouth opening. Plankton consumed by fish changes with the growth and morphology changes.