: Life history stages of Pleurogonius malaclemys were investigated in wild populations of the eastern mudsnail ( Ilyanassa obsoleta ) and diamondback terrapin ( Malaclemys terrapin ) in New Jersey, New York, and Rhode Island between 2011 and 2015, and laboratory experiments investigating the settling preference of metacercarial cysts of P. malaclemys were conducted. Cysts of P. malaclemys were found on mudsnails on the north and south shores of Long Island, New York and in Rhode Island, approximately 280 km farther north than previously reported. The cysts were found on mudsnails year round, but cyst prevalence increased during the summer months, reaching maximum levels (∼70%) in November. Nearly 58% of Jamaica Bay, New York mudsnails had cysts; mean intensities were 2.63 cysts/mudsnail. Although cyst prevalence was high, only 11 mudsnails (0.28%) were found to have the internal redial stages of P. malaclemys, the stage of infection preceding external cysts. In addition to mudsnails, P. malaclemys could encyst on other biological substrates, including common terrapin prey species. The majority of wild adult terrapins from Stone Harbor, New Jersey were infected with the adult stage of P. malaclemys (80.30%, x¯ = 36.36 trematodes/terrapin, n = 66). Juvenile terrapins were experimentally infected with P. malaclemys and on average 22.5% of the consumed cysts successfully developed into adult trematodes. Studies on the life cycle of P. malaclemys are important because previous research has shown that the frequency of cysts of P. malaclemys on mudsnails can be used as an indirect measure of terrapin abundance.