Quahog parasite unknown (QPX) is a protistan parasite that causes significant mortalities among hard clams, Mercenaria mercenaria, in the northeastern United States and Canada. This pathogen has been successfully isolated from clams from different geographic locations, and in vitro cultures are being used in investigations of the parasite's genetic makeup, virulence and environmental tolerances. Many of these investigations require an easily reproducible, quantitative method to rapidly measure QPX cell viability and proliferation. Therefore, a fluorometric microplate technique was developed using fluorescein diacetate (FDA) as an indicator of cell viability. The developed technique provides a good estimate of the biovolume of live QPX cells. Fluorescent signals are correlated with the number and the size of individual QPX cells. Optimal experimental conditions included an FDA concentration range of 30–50 μM with an incubation period of 15–30 min in the dark. This FDA assay was used to investigate the dynamics of in vitro growth of QPX. Results showed that the growth dynamic is different among QPX isolates. For instance, exponential growth lasted for 1 week in two QPX isolates cultured from clams collected from New York and Massachusetts; whereas a third isolate (from New York) grew exponentially during 2 weeks under similar experimental conditions. While there are limitations to in vitro studies that must be recognized, research using cultured QPX cells is indispensable and will probably lead to significant progress in our knowledge of the biology and physiology of this parasite.