In this study we evaluated the responses of the tropical micro-crustacean, Daphnia lumholtzi, Environmental Management and Sustainable Development ISSN 2164-7682 2016 www.macrothink.org/emsd 171 to zinc (Zn). For a more environmentally realistic assessment on the Zn toxicity, water sample collected from Saigon River was used as medium for the daphnids and Zn was spiked into field water for exposures. River water was physically and chemically measured and analyzed, and the sufficient water quality was confirmed prior to be used for the acute and chronic experiments. Metal concentrations in the test water were determined with an inductively coupled plasma/ mass spectrometry (ICP/MS). In the acute test implementation, Zn concentrations were from 67 -360 µg/L. In the chronic experiments, the animals were incubated in control (filtered field water only) or filtered field water added with Zn (28, 46, and 75 µg/L) over the period of 14 days. The life history traits of daphnids including the survival, maturation, reproduction were daily observed. The results showed that the value of median lethal concentrations (48h-LC 50 ) was 136 µg Zn/L (95% CI = 107 -165 µg Zn/L). Chronic exposures revealed that Zn caused the survival reduction, delayed maturity age and reproduction decrease. Our study suggested D. lumholtzi as a very good freshwater aquatic zooplankton for assessment on the toxicity of metals in tropics. Besides, the Vietnam guideline values regarding Zn should be re-considered and adjusted for the aquatic ecological protection.