Algal biomass produced in an outdoor pilot plant facility for the treatment of the liquid phase of pig slurry was nutritionally evaluated together with Chlorella vulgaris 211/le produced axenically in defined media in laboratory chemostats. Data from amino acid analyses showed that the sulphur-containing amino acids were limiting. Digestibility, net protein utilisation and biological values of the algal products were assayed. In vivo and in vitro enzymic digestions were monitored by transmission electron microscopy. Although the cell wall was only partially digested, the cell contents, and especially the pyrenoid, were extensively digested in several treatments. The most indigestible components of the cell may be the membranes of the chloroplast. It is suggested that the absence of sporopollenin from the cell wall of this alga may be responsible for the high nitrogen digestibility of the laboratory-produced material. The waste-grown algal biomass contained appreciable amounts of algal species other than C. vulgaris 211/le which may have contributed to the lower digestibility of this material.