Natural diet of zooplankton from Clavellinos reservoir (oligo-mesotrophic) was studied. A total of 631 specimens (cyclopoid copepods, nauplii, cladocerans, and rotifers) were analyzed with Hoyer’s medium, which cleared animals but did not alter the gut contents. Detailed analysis of gut contents showed that particulate material was the main food item for zooplankton in the reservoir. Diatoms and green algae were consumed in high proportion. Significant differences in diets were detected between rainy and dry seasons for nauplii, Ceriodaphnia cornuta, Diaphanosoma spinulosum, Brachionus falcatus and Plationus patulus. Adult copepods (Thermocyclops decipiens) showed the highest values of the trophic diversity index, while Keratella sp. (Rotifera) and the cladoceran Moina micrura showed the lowest values. Further studies should focus on the origin of the consumed particulate material.