Some groups of ciliates specialized to live in or on other organisms. Quantitative data on their spatial distribution, frequency of occurrence as well as the range of organisms inhabited are often not well known. In order to investigate the distribution and prevalence of the presumably parasitic ciliate Protophrya ovicola inhabiting periwinkles, we conducted a field sampling in autumn 2017 at 20 locations from seven regions (France, the Netherlands, Sweden and the United Kingdom) along the shorelines North East Atlantic. In addition, we present a comprehensive data set of P. ovicola found in the rough periwinkle Littorina saxatilis from the North West Atlantic. We found P. ovicola in all seven regions and in four species of periwinkles investigated. Our results indicate a wide spatial distribution of the ciliate within the North East Atlantic and a wide range of Littorina species being inhabited. Although highest frequency of occurrence and abundance were observed in females of L. saxatilis, where P. ovicola inhabits the brood pouch of the live-bearing snail, also the mantle cavities of males of L. saxatilis and of females and males of the other three species were inhabited by the ciliate. The ciliate inhabits L. saxatilis with a higher prevalence in the North West Atlantic than in the North East Atlantic. Given the broad distribution and relatively high frequency of occurrence of P. ovicola, further research into the potential effects on inhabited snails is warranted.