Aims: To investigate the effects of temperature, pH, water activity (a(w)) and CO2 concentration on the growth of Rhizopus oligosporus NRRL 2710. Methods and Results: Hyphal extension rates from mycelial and spore inocula were measured on media with different a(w) (similar to 1.0, 0.98 and 0.96) and pH (3.5, 5.5 and 7.5) incubated at 30, 37 or 42degreesC in atmospheres containing 0.03, 12.5 or 25% (v/v) CO2. The effects of environmental conditions on hyphal extension rate were modelled using surface response methodology. The rate of hyphal extension was very sensitive to pH, exhibiting a pronounced optimum at pH 5.5-5.8. The hyphal extension rate was less sensitive to temperature, a, or CO2, exhibiting maximum rates at 42degreesC, a(w) similar to 1.0 and 0.03% (v/v) CO2. Conclusions: The fastest hyphal extension rate (1.7 mm. h(-1)) was predicted to occur at 42degreesC, pH 5.85, a(w) similar to 1.0 and 0.03% CO2. Significance and Impact of the Study: The present work is the first to model the simultaneous effects of temperature, pH, a(w) and CO2 concentration on mould growth. The information relates to tempe fermentation and to possible control of the microflora in Tanzanian cassava heap fermentations.