Representative protein fractions were isolated from Chlorella vulgaris, Anabaena cylindrica, Navicula pelliculosa, and Tribonema aequale, which are members of four different algal classes. Quantitative determinations of the amino-acid compositions of these proteins were made, and a comparison of the results shows that a close similarity exists for the proteins of all four species. Subject to certain limitations, the tentative conclusion is reached that the protein complements of all the algae are similar. The compositions of the algal proteins are also compared with those of other plant proteins compiled from data available in the literature.