Aims Brown ring disease (BRD) is an infection of the Manila clamRuditapes philippinarumdue to the pathogenVibrio tapetis. During BRD, clams are facing immunodepression and shell biomineralization alteration. In this paper, we studied the role of pH on the growth of the pathogen and formulated hypothesis on the establishment of BRD byV. tapetis. Methods and Results In this study, we monitored the evolution of pH during the growth ofV. tapetisin a range of pH and temperatures. We also measured the pH of Manila clam haemolymph and extrapallial fluids (EPFs) during infection byV. tapetis. We highlighted thatV. tapetismodulates the external pH during its growth, to a value of 7 center dot 70. During the development of BRD,V. tapetisalso influences EPFs and haemolymph pHin vitroin the first hours of exposure andin vivoafter 3 days of infection. Conclusions Our experiments have shown a close interaction betweenV. tapetisCECT4600, a pathogen of Manila clam that induces BRD, and the pH of different compartments of the animals during infection. These results indicate that the bacterium, through a direct mechanism or as a consequence of physiological changes encountered in the animal during infection, is able to interfere with the pH of Manila clam fluids. This pH modification might promote the infection process or at least create an imbalance within the animal that would favour its persistence. This last hypothesis should be tested in future experiment. Significance and Impact of the Study This study is the first observation of pH modifications in the context of BRD and might orient future research on the fine mechanisms of pH modulation associated with BRD.