Growth efficiency () and specific maintenance rate constant ( ) were determined in continuous cultures of the cyanobacterium Gomont, in light energy-limiting conditions, according to the formula = - in which is the specific light energy uptake rate. Values of the efficiency factor varied with irradiance from 0·23 at 0·5 W m to 0·05 at 40 W m. The specific maintenance rate constant was 0·001 h. The culture pH value influenced and A diurnal light/dark cycle with a 16 h photoperiod did not affect either of the two parameters, and The relationship between growth rate and light energy uptake rate, embodied in the above formula, was also valid for nitrogen (nitrate)-limited cultures.