Different kinetic models have been proposed to describe the elimination of hydrophobic volatile organic compounds (VOCs) by fungal biofiltration. In this process the ratio of the initial substrate concentration (Cpb0) to the initial biomass (X0) has been shown to influence the cell yield. This paper presents a study of the effect of the Cpb0/X0 ratio on bserved cell yield (Yobs) in a fixed bed batch system (microcosm) using a gaseous carbon source, as an approximation to its application in the fungal biofiltration of hydrophobic VOCs. Essays were carried out in fixed-bed microcosms using the filamentous fungus Fusarium solani as a biological agent and n-pentane as a carbon and energy source. The results indicated that Yobs in the gas phase is inversely proportional to the Cpb0/X0 ratio, with values of 0.9 to 0.35 gbiomass g-1 pentane being obtained when the Cpb0/X0 ratio is changed from 0.1 to 1.0 gpentane g-1 biomass. The results indicate that more than 60% of n-pentane was consumed due to energy spilling, and that strong dissociation of catabolism from anabolism occurred at higher Cpb0/X0 ratios.