Stress relaxation and constant rate of strain measurements were made on the cell walls of the green algae Nitella, Acetabularia, and Penicillus. These plants have, as theirmain structural polysaccharide, cellulose, mannan, and xylan respectively. The stress relaxation measurements were made in the temperature range 0 to 50 °C for the interval 0.02 to 103 s. A limited number of constant rate of strain curves were also obtained for comparison with theoretical curves deduced from the relaxation data. All three algae exhibit typical fibrous behaviour but Acetabularia could not be mechanically conditioned to give reproducible results. The time-temperature correspondence of the data and its relevance to the molecular organization of the cell wall are discussed. Structural changes with temperature, deduced in previous work from rigorous matching of the relaxation curves at different temperatures, are now thought not to occur. The matching procedure only appears to work because the curves lack distinctive features on which to base the superposition. Comparison of stress relaxation and constant rate of strain data show that the cell walls of these algae exhibit non-linear viscoelastic behaviour and apparent changes in the quasi-static modulus with temperature can be explained satisfactorily in terms of reaction rate theories for viscoelastic processes.