The first goal of this study was to measure geographic variation in the abundance of Microphallus turgidus in the grass shrimp Palaemonetes pugio and to determine whether salinity at collection localities and parasite abundance are related. To do so, grass shrimp were collected at multiple localities along the Altamaha and Ogeechee rivers that varied in salinity from oligo- to polyhaline. We found a positive linear relationship between salinity and mean parasite abundance. The second goal of the study was to measure temporal variation in populations of M. turgidus in the grass shrimp P. pugio and Palaemonetes vulgaris to determine whether a relationship between temporal changes in parasite population size and variation in salinity or water temperature exists. To do so, grass shrimp were collected twice each season over the course of approximately 2.5 yr at 1 locality each on 3 tidal rivers in southeast Georgia: the Ogeechee, Broro, and Altamaha rivers. Variation between seasons in the abundance of M. turgidus in P. vulgaris was not significant. In P. pugio collected at the Altamaha and Ogeechee river localities, parasite abundance and, to some extent, parasite prevalence varied over the course of the sampling period. Correlation between water temperature and parasite abundance in P. pugio shrimp from both the Altamaha and Ogeechee river collection localities and between salinity and parasite abundance at the Altamaha River locality was slight but significantly positive. Neither relationship was observed in P. vulgaris shrimp. The results provide evidence that geographic differences in salinity might explain some of the variation in parasite abundance from one locality to another and that temporal variation in water temperature and, to some extent, salinity might also affect M. turgidus populations in P. pugio.