Large amounts of seagrass, Posidonia australis, wash onto beaches in South Australia each year, causing substantial environmental problems. It was of interest to assess the potential for an economic use of this seagrass-such as for animal nutrition. Structural constituents of P. australis (green, freshly deposited, and both washed and unwashed samples from dried deposits on the beach) were examined and compared. Glucose, galactose, and mannose were the dominant sugars (>10 g kg-1 of dry matter) in the soluble fraction of nonstarch polysaccharides in all seagrass forms. The content of the insoluble constituents of the nonstarch polysaccharides was significantly higher than soluble nonstarch polysaccharide constituents (P < 0.01). Data showed that the major constituents of the Posidonia cell wall are cellulose and lignin (190-209 and 145-154 g kg-1, respectively). The crude protein content of Posidonia ranged from 54 to 61 g kg-1. Results showed no biologically significant compositional differences between the four different forms of seagrass tested. Dry, unwashed seagrass, which is readily available in large quantities and easily harvested, may have potential as a foodstuff for ruminant animals.