The microcolony technique enables the effects of several atmospheric conditions on fungal growth to be studied by measuring the radius of the colony, while excluding effects of those conditions on germination of the sporangiospores. Various concentrations of oxygen and carbon dioxide in the gas environment were found to influence growth of Rhizopus oligosporus on malt extract/soya peptone/agar. The maximum radial growth rate was 1.48 mm/h and the maximum specific growth rate was 0.109 h(-1) at 30 degrees C. Oxygen became limiting below 1% (v/v), but growth remained possible at levels of 0.001% oxygen. Carbon dioxide stimulated growth at limiting oxygen levels. The specific growth rate increased from 0.043 h(-1) at 0.5% (v/v) oxygen and 0% (v/v) carbon dioxide to 0.096 h(-1) at 0.5% (v/v) oxygen and 5% (v/v) carbon dioxide. A mixture of 0.5% (v/v) oxygen and 35% (v/v) carbon dioxide inhibited growth. Delay of sporangiospore germination due to low (less than 0.001%) amounts of oxygen was not observed with the techniques used. Fungal activity in a rotating drum fermenter was more strongly affected by low levels of oxygen than was biomass formation on model media. High concentrations of carbon dioxide inhibited growth in the rotating drum fermenter at non-limiting levels of oxygen. It is concluded that aeration and heat removal are both essential aspects of optimization of large-scale solid-substrate bioreactors with Rh. oligosporus.