When Aspergillus nidulans was cultured on a 1 cm2 dialysis membrane squares overlaying minimal medium, the mycelium expanded beyond the membrane boundary after 48h at 37ºC. When the membrane was removed, the underlying clear patch was not recolonized despite nutrient supplementation. More detailed experiments showed that growth inhibition is proportional to the length of time at which pre-growth had taken place on the membrane, reaching 100% inhibition after 72h of pre-growth. Chemical analysis of the ’spent’ medium, combined with an inhibition bioassay revealed that the inhibiting factor was a mixture of sodium and potassium bicarbonate. Under nitrate based growth, progressive medium alcalinisation occurs resulting in fixation of CO2 as bicarbonate beyond pH 6.3. At concentrations ranging between 0-50mM, bicarbonate exerts a progressive effect on extension rate (Kr) as well as the hyphal density of mycelia. The effect of bicarbonate is fungistatic, resulting in progressively higher number of cells arrested at G1. Their survival in this state is dependent on the provision of nutrients. Upon dilution of bicarbonate, viable cells resume growth normally. The results will be discussed in the context of bicarbonate as an endogenous regulator of within the mycelium.