Perkinsus marinus, a protistan parasite of the Eastern oyster (Crassostrea virginica), has been a contributing factor to the severe decline of farmed and wild oysters on the East Coast of the United States and the Gulf of Mexico. Recent findings suggest that the pallial organs (mantle, gills) of the Eastern oyster are an important portal of entry for the parasite. In this case, the first contact P. marinus has with its host takes place in mucus covering the mantle and gill tissue. This study consisted of several experiments to investigate the effect of oyster pallial mucus on the growth, expression of virulence genes (pmSOD1, pmSOD2 and pmSUB), protease production and infectivity of P. marinus. In each experiment, P. marinus grown in pallial mucus (mantle, gill or both) was compared to P. marinus grown in media supplemented with seawater (control) and other experimental media such as oyster plasma or digestive extracts since the digestive tract is traditionally considered as the main portal of entry for the parasite. P. marinus grown in media supplemented with mantle mucus showed a significantly higher growth rate than cultures added with other supplemental extracts, while cultures grown in gill mucus promoted a higher protease production. No differences were noted in the expression of virulence-related genes between cultures supplemented with mantle or gill mucus as compared to those added with seawater, however those grown in digestive extract or plasma showed a down-regulation compared to control (seawater) cultures. Lastly, challenge experiments showed that parasite cultures grown in pallial mucus caused severe early mortality of oysters coupled with high infection intensities, whereas oysters injected with cultures grown in seawater showed minimal mortalities and no mortalities were found in oysters injected with cultures supplemented with digestive extract during the four week experiment. These results demonstrate that oyster mucus plays a significant role in the pathogenesis of P. marinus by enhancing the proliferation and the infectivity of this devastating parasite.