SUMMARYField observations ofMetiola argentina, parasitic on the large sedgeMachaerina angustifolia, indicate that distribution of this fungus on Oahu is determined primarily by rainfall. The lowest elevation at which the fungus develops coincides closely with the isohyet for 250 cm/yr rainfall. Although the range of the host plant extends well below this, abundant infections occur only in areas where rainfall equals or exceeds this amount. When ascospores ofM. argentinaandM. palmicolawere plated singly on agar media, approximately 4–10% germination was obtained. Growth ceased after emergence of the germ tube. Similar levels of germination were obtained with ascospores incubated in water drops, indicating that exogenous nutrients or contact with the host are not required for germination, but that the parasitic relationship is apparently essential for continued growth of the fungus. The results indicate a low level of germinability in the naturally occurring ascospore population.