Semicontinuous fungal fermentation of lactic acid (LA) by Rhizopus arrhizus was performed using a periodical bleed and feed (PBF) operation in a stirred tank bioreactor with a volume of 4.5 dm(3). LA productivity up to 2.91 kg m(-3) h(-1) and yield up to 75.3% were achieved. Using PBF cycles, fermentation was prolonged to 240 h with an average LA productivity of 2.31 kg m(-3) h(-1) and a total yield of 67.3% at a mean pH of 5.30. The most effective production of LA required filamentous morphology of the biomass. When such morphology changed to pellets biomass growth stopped, the LA productivity decreased to 1.63 kg m(-3) h(-1) and the yield decreased to 62.6%. The filamentous morphology was successfully maintained for 152 h when the inoculation was with spores. CaCO3 initially used for pH neutralisation did not dissolve well since it was entrapped by biomass. It was successfully replaced by 25% ammonia. Long-time operation below pH 5 decreased LA productivity. Accumulating biomass in the fermenter did not increase the LA productivity. With an increase in the biomass growth rate, the LA productivity increased. The on-line extraction or pertraction of LA by trialkylamines demands a pH value of feed below 4. This can be achieved for a short time by interruption neutralisation at the end of each PBF cycle without a decrease in LA productivity after the restoration of the optimal pH value. (C) 2003 Elsevier Science Ltd. All rights reserved.