The microalga, Chlorella vulgaris, was immobilized in carrageenan gel. The growth of C. vulgaris in the carrageenan matrix exhibited a one day longer lag period when compared with the free cells. However, growth of the immobilized cells in the log phase was nor hindered, as shown from the specific growth constant (k) which was virtually the same as that of the free cells (0.397 vs 0.362 d(-1)). The rare of chlorophyll synthesis in the immobilized cells was double that of the free cells, compensating for the self-shading or gel-screening effect. The depletion of the nutrient ions, nitrate and phosphate, from the medium showed no significant difference between the immobilized and free cells. For the activity of nitrate reductase, though the immobilized cells had assimilated a similar amount of nitrate as the free cells, their kinetic profiles were different. The nitrate reductase activity increased more slowly than that of the free cells and had a maximal level about half that of the free cells (300 vs 700 x 10(6) mu mol NO2 mu g protein(-1) h(-1)). The decline in the nitrate reductase activity in the immobilized cells was also less drastic than that in the free cells upon the depletion of nitrate ions in the medium. This study provided evidence that carrageenan gel had no adverse effect on the growth and physiology of the immobilized microalga. The enzyme activity was also conserved in the immobilized cells. Carrageenan could thus be an alternative gel matrix for immobilizing microalgae.