Hydrogen production rates by Anabaena sp. strain TU37-1 obtained after an initial 1-day incubation period were approximately 70 to 80 and 3 to 9 mumol (mg chl)(-1) h(-1) under argon and nitrogen atmospheres, respectively. Hydrogen production under argon was not enhanced by addition of carbon dioxide, but was enhanced to some extent under nitrogen by increasing the initial carbon dioxide concentration. Rates of hydrogen and oxygen production during the initial 7-hour period were 15 and 220 mumol (mg chl)(-1) h(-1), respectively, in vessels with 18.5% initial carbon dioxide. Hydrogen production under nitrogen was enhanced by addition of carbon monoxide (M). The rate obtained from the initial 1-day incubation period was about 40 mumol (mg chl)(-1) h(-1), which corresponded to about 60% of that under argon. On the basis of these observations, a possible strategy for hydrogen production by nitrogen-fixing cyanobacteria under nitrogen in the presence of carbon monoxide is indicated.