The direct method based on inlet- and outlet-gas concentrations in submerged fermentation (SmF) was adopted for estimating overall oxygen-transfer coefficients (K-L a) in a packed-bed solid-state fermenter. The variation in K-L a during fermentation at different air-flow rates was studied. The values of K-L a were higher than those found in SmF. The moisture content was found to affect oxygen-transfer rates and coefficients. The value of K-L a increased with an increase in air-flow rate in the range 0-15 litre/min but decreased at higher air-flow rates (15-25 litre/min). This could be attributed to the decrease in the moisture content of the substrate at high flow rates.