Specifically radiolabeled [14C-lignin] lignocellulose and uniformly [U-14C] lignocellulose from the salt marsh grass Spartina alterniflora were incubated with the ascomycete Phaesphaeria s spartinicola. This fungus is the predominant one found on decaying standing dead S. alterniflora leaves in the salt marsh ecosystem. After 45 days of incubation at 20°C, 3.3% of the lignin moiety was mineralized to 14CO2 and 2.7% solubilized to DO14C. Mineralization of the polysaccharides was seven times faster than that of the lignin. About 22% of the radioactivity was evolved as 14CO2 but merely 4% was solubilized to DO14C within the incubation time. Experiments monitoring the ergosterol content of the mycelium incubated with S. alterniflora plant material were done to elucidate the carbon conversion efficiency of the fungus as well as the influence of the cinnamyl phenols p-coumaric and ferulic acid on lignocellulose degration. After 21 days of incubation, P. spartinicola showed a growth yield of 0.45 and 0.38 with and without the additional cinnamyl phenols, respectively. Grown on unextracted S. alterniflora the fungus caused a loss of organic plant material of about 50% with a corresponding growth yield of 0.38 during the incubation period. Investigation of cupric oxide oxidation products of sound and degraded lignocellulose revealed a preferential utilization of the syrinyl and cinnamly phenols compared with vanilly phenols.