A field monitoring of hemocyte parameters of the Manila clam Ruditapes philippinarum was conducted from July 2004 to September 2005 in Gulf of Morbihan (France), in order to assess (1) the factors controlling the hemocyte parameters of the Manila clam and (2) their relative contribution to the overall variability of these parameters. Monthly, sixty clams were sampled and total hemocyte count (THC), granulocyte and hyalinocyte counts, phagocytosis, phenoloxidase specific activity, length, flesh dry weight, and condition index were measured individually. Perkinsus sp. infection and Brown Ring Disease symptoms were also monitored. Temperature and trophic resource were also monitored. Results indicate that temperature controls granulocyte counts and subsequently THC. Other environmental factors had no direct influence on the measured hemocyte parameters. Almost all measured parameters were significantly affected by size/age and condition index. There were poor relationships between both pathologies and hemocyte parameters presumably because of low infection intensities. Nevertheless, high Perkinsus sp. infection intensity significantly increased total and granulocyte counts and decreased phagocytosis. An interesting result of this study is that the measured biotic and abiotic factors poorly contribute to the explanation of the total variability of hemocyte parameters. Granulocyte concentration was the best explained parameter. However, only 16.4% of its variance was explained by cumulating temperature, length, condition index and Perkinsus sp. infection effects. This study emphasizes the need for a better understanding of hemocyte functions and the factors modulating these functions.