Ceriodaphnia dubia is a species of water flea in the class Branchiopoda, living in freshwater lakes, ponds, and marshes in most of the world. They are small, generally less than 1 millimetre (0.039 in) in length. Males are smaller than females. Ceriodaphnia dubia move by using a powerful set of second antennae. Ceriodaphnia dubia is used in toxicity testing of wastewater treatment plant effluent water in the United States.