In 2000, three subspecific groupings within the fungal pathogen Claviceps purpurea (Fr.) Tul were discovered. These groups are habitat specialized, where group 1 (G1) is found on terrestrial grasses, G2 is found in freshwater environments and G3 is found in salt marsh habitats. An intraspecific comparison of 43 G3 isolates, seven G1 isolates, and two G2 isolates using random amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP) analysis support the recognition of three discrete groups within C. purpurea and showed high genetic variability between groups, with only 1.8% of markers shared between all isolates. For isolates collected in salt marshes (G3), there was less diversity and four primary clusters were strongly supported by bootstrap: 1) Northeastern USA plus Western Europe, 2) Southeastern USA, 3) Argentina, and 4) Pacific Coast ofthe USA. Distich/is spicata was identified as the first non-Spartina host to G3 C. purpurea. Maritime C. purpurea (G3) is present at very high rates on the native Spartina foliosa on the outer coast of the San Francisco Bay, California. Invasive S. alterniflora is host to maritime ergot in its native range (Atlantic Coast of the USA) but along with hybrid Spartina (S. alterniflora x S.foliosa), is almost ergot free in the San Francisco Bay. Rates of infection were consistent over a three-year period and greenhouse inoculations showed no differences in susceptibility among Spartina plants. In the San Francisco Bay Area, the pathogen caused a 68% reduction in seed production and negatively effected seed quality by reducing seed weight. All three sub-specific groups of C. purpurea were found on grasses in Willapa Bay, Washington. Phenetic analysis of AFLP markers suggests that maritime C. purpurea in Washington is most closely related to Southeastern USA isolates. More research is necessary to determine if a climate adapted isolate would be an effective biological control agent for invasive Spartina. Host resistance cannot explain the low incidence of ergot throughout Willapa Bay. Terrestrial grasses, specifically cereal grains, are susceptible to G3 C. purpurea under controlled conditions. In natural systems, the host range seems to be more limited.