Experiments showed that kumara and tare were excellent substrates for citric acid production by solid-substrate fermentation using Aspergillus niger. Conversely, potato was a poor substrate, although it supported profuse fungal growth. A kinetic analysis of citrate production from kumara showed an overall reactor productivity of 0 . 48 g citrate/kg wet weight kumara per hour with a yield of 0 . 54 on a weight basis (g citrate produced/g starch used). Maximum citrate production rates were observed after 2 to 3 days of fermentation, while the fungal growth rate was still high. The optimum moisture content of the kumara for citrate production was 65% (w/w) or above, while metal ions were shown not to be inhibitory to the process.