| Summary: | <i>Mucochytrium quahogii</i>, commonly known as QPX (Quahog Parasite Unknown), is the causative agent of QPX disease in hard clams (<i>Mercenaria mercenaria</i>), but poor understanding of the relationship between host and pathogen has hindered effective management. To address this gap in knowledge, we conducted a two-year study quantifying the distribution and abundance of <i>M. quahogii</i> in hard clam tissue, pallial fluid, and the environment. <i>M. quahogii</i> was broadly distributed in clams and the environment, in areas with and without a known history of QPX disease. <i>M. quahogii</i> in clams was not strongly related to <i>M. quahogii</i> in the environment. <i>M. quahogii</i> was always present in either the tissue or pallial fluid of each clam, with an inverse relationship between the abundance in the two anatomical locations. This study suggests that the sediment–water interface and clam pallial fluid are environmental reservoirs of <i>M. quahogii</i> and that there is a host-specific relationship between <i>M. quahogii</i> and the hard clam, supporting its classification as a commensal, opportunistic pathogen. There appears to be minimal risk of spreading QPX disease to naïve clam populations because <i>M. quahogii</i> is already present and does not appear to be causing disease in hard clam populations in locations unfavorable for pathogenesis.
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