Although water level fluctuations and related environmental variations occurring within reservoirs have a considerable effect on the zooplankton community, studies on zooplankton dynamics of reservoirs are very limited. In the present study, we aimed to elucidate the diurnal vertical migration of zooplankton in relation to the concentration of physicochemical factors and chlorophyll a content in a recently formed water reservoir (Tahtali Reservoir, Kocaeli, northwestern Turkey). During the sampling period 20 zooplankton taxa were identified, which were mostly indicative of eutrophic waters, as confirmed by the physicochemical factors of the water in the reservoir. Rotifera was the most abundant group, followed by Copepoda and Cladocera. Statistical analyses indicated that all the zooplankton species showed a similar distribution pattern throughout the study. They declined in abundance in the water column from the surface to a depth of 6 m, and then increased relatively again towards the deepest part of the water (9 m). Abiotic factors, chlorophyll a, time of the day, and depth significantly impacted the abundance of zooplankton, whereas they jointly had no effect. The results suggested that abiotic factors and chlorophyll a were controlling factors on the vertical distribution pattern of the zooplankton in the Tahtali Reservoir.