Perfluoroalkyl substances (PFAS) such as perfluorooctane sulfonic acid (PFOS) and perfluorohexane sulfonic acid (PFHxS) are of high concern due to their wide application in consumer and industrial products, extreme persistence, occurrence in the environment and their bioaccumulation and toxicity to humans and animals. The present study was designed to assess the chronic toxicity of PFOS and PFHxS individually and their binary mixture (PFOS+ PFHxS) to waterflea, Ceriodaphnia dubia. Chronic toxicity tests were conducted following standard USEPA protocol. Survival, reproduction and growth were measured during eight days exposure to PFOS and PFHxS at 5, 15 and 50 μg/L, both individually and in mixtures. The measured concentration was always ≥90% of the nominal concentrations. PFOS and PFHxS individually at 5 at µg/L did not affect the survival and reproduction of C. dubia. PFOS was more toxic to C. dubia than PFHxS at 15 and 50 μg/L. All binary combinations exhibited less than additive or additive effects with PFOS concentration influencing the toxicity in binary mixtures. Our results highlight the complexity of PFAS toxicity and demonstrate the need to better understand PFAS toxicity in mixtures.