Objectives To improve H-2 production, the green algae Chlamydomonas reinhardtii cc849 was co-cultured with Azotobacter chroococcum. Results The maximum H-2 production of the co-culture was 350% greater than that of the pure algal cultures under optimal H-2 production conditions. The maximum growth and the respiratory rate of the cocultures were about 320 and 300% of the controls, and the dissolved O-2 of co-cultures was decreased 74%. Furthermore, the in vitro maximum hydrogenase activity of the co-culture was 250% greater than that of the control, and the in vivo maximum hydrogenase activity of the co-culture was 1.4-fold greater than that of the control. In addition, the maximum starch content of co-culture was 1400% that of the control. Conclusions Azotobacter chroococcum improved the H-2 production of the co-cultures by decreasing the O-2 content and increasing the growth and starch content of the algae and the hydrogenase activity of the co-cultures relative to those of pure algal cultures.