Transformation of marsh grass, Spartina alterniflora, to detritus is an initial energy transfer step in the coastal Louisiana estuarine ecosystem. Spartina is systematically attacked by a selective mycota throughout its development and decomposition. Fungi include, among others, species of Fusarium and Cephalosporium as well as representatives of the marine taxa Lulworthia and Leptosphaeria. Molds colonizing external plant surfaces differ from those isolated within the culm. Fungal attack is correlated with seasonal development and subsequent decomposition of the plant. A large yeast biomass, notably sporogenous taxa Pichia spartinae and Kluyveromyces drosophilarum, is prevalent in the oxidized portions of the Spartina rhizosphere and within the peripheral tissue and intercellular spaces of the culm. These species, with stron B-glucosidase activity, reach maximal populations during dieback of Spartina. A mutualistic yeast/mold association in turnover of plant substrates is suggested.