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Аннотация A field experiment was conducted in 2009 using Lake Ontario water in a continuous flow through system to determine the impact of low pH on dreissenid mussel (zebra mussel, Dreissena polymorpha Pallas, 1771, and quagga mussel, Dreissena rostriformis bugensis Andrusov, 1897) settlement and survival in calcium rich waters. Raw water containing veligers was pumped to the field laboratory where the incoming water was divided into four streams. Three of the streams had the pH adjusted using phosphoric acid to pH 7.3, 7.1 and 6.9. The fourth stream was used as a control. Three replicates of each pH resulted in 9 treatment tanks and three control tanks. Three bags of caged adults were placed in each tank. Adult mortality of almost 40% was recorded at a pH of 6.9 after 10 weeks of exposure. Analysis of the weight/length relationship of adult mussels confirmed that for any given length the mussels were significantly lighter at all three pH levels when compared to the controls. Visually, erosion and perforation of the shells was noted, leading to the conclusion that the loss in weight was primarily due to loss of calcium from the shells of the adults. The visual loss of calcium was the greatest at a pH of 6.9. New settlement was essentially prevented at a pH of 7.1. Based on these results, downward adjustment of pH in calcium rich waters may be a viable treatment for prevention of dreissenid fouling in industrial cooling water systems and raw water conveyances.