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Ichthyophthirius multifiliis is a unicellular freshwater fish parasite and the causative agent of the globally distributed white spot disease. The fitness of the parasite depends on available hosts and abiotic factors such as temperature, salinity and pH. With climatic change these abiotic factors may be altered, thereby influencing the health of the parasite. In this study, the tolerance towards different pH values (2-11) was investigated on a Nordic strain of the parasite by recording tomont survival, release of theronts, theront size and theront survival. Tomonts were able to survive and release theronts in pH 5-10, however the number of released theronts was significantly lower at high and low pH. Theronts produced at pH 8 and exposed to the different pH values survived at pH 4-10 for 1 h, which may be sufficient time for the parasite to locate and infect new hosts. The release of theronts was slower at pH 10, and the size of theronts developed at higher pH was significantly increased (up to 73.5 µm in length). In conclusion, our study showed that the free-living stages of I. multifiliis were capable of surviving at a pH from 5 to10, and that high pH had an effect on the morphology and release of the parasites., (Copyright © 2020 Australian Society for Parasitology. Published by Elsevier Ltd. All rights reserved.)