Kumara, a starch-containing root crop grown extensively in New Zealand, has been used as a substrate for citric acid production using Aspergillus niger in solid-state fermentation. When the process was operated in a packed-bed reactor, the bed loading was the most important operational parameter. The airflow rate and substrate particle size were also important but their net effects varied depending on other parameters; thus, a high airflow rate was conducive to citric acid production except under certain operating conditions (e.g., large substrate particle size) where it appeared to inhibit fungal growth possibly due to shear stress. A kinetic analysis of the system showed an overall reactor productivity of 0.82 g citrate kg(-1) wet weight kumara h(-1) which is twice that observed in flask culture. In this respect, the results in the packed-bed reactor were superior to published results in flask culture probably due to the improved aeration that was achieved.