The free living Copepods are considered as Zooplankton and consume as food by fishes. In the present study diurnal fluctuations in the population density of copepods in relation to temp., DO, pH, Acidity and Alkalinitywere studied in Yassbolagh Dam, which is situated in north west of Arak city. The study covered four diel periods in three months of summer, 2008. The copepods are a sub class of the class Maxillipoda, sub phylumcrustacean and phylum Arthropoda, and in this study they were represented by 8 genus, 11 species, and 3 orders(Cyclopoida, Calanoida and Harpacticoida) respectively. Maximum papulation density observed in D.lintoni. The high populatin density of copepoda in 12 and 18 possibly is due to their diurnal habitat, more water temp. and relatively moderate o2consentration.
