Hairy roots have the potential to produce a variety of valuable small and large molecules The mist reactor is a gas phase bioreactor that has shown promise for low-cost culture of hairy roots Using a newer disposable culture bag mist reactor performance was studied with two species Artemisia annua L and Arachts hypogaea (peanut), at scales from 1 to 20 L Both species of hairy roots when grown at 1 L in the mist reactor showed growth rates that surpassed that in shake flasks From the information gleaned at 1 L, Arachzs was scaled further to 4 and then 20 L Misting duty cycle, culture medium flow rate, and timing of when flow rate was increased were varied In a mist reactor increasing the misting cycle or increasing the medium flow rate are the two alternatives for increased delivery of liquid nutrients to the root bed Longer misting cycles beyond 2 3 min were generally deemed detrimental to growth On the other hand, increasing the medium flow rate to the sonic nozzle especially during the exponential phase of root growth (weeks 2-3) was the most important factor for increasing growth rates and biomass yields in the 20 L reactors A hypogaea growth in 1 L reactors was mu = 0 173 day(-1) with biomass yield of 12 75 g DW L-1 This exceeded that in shake flasks at mu=0 166 day(-1) and 11 10 g DW L-1 Best growth rate and biomass yield at 20L was mu=0 147 and 7 77 g DW L-1, which was mainly achieved when medium flow rate delivery was increased The mist deposition model was further evaluated using this newer reactor design and when the apparent thickness of roots (+hairs) was taken into account, the empirical data correlated with model predictions Together these results establish the most important conditions to explore for future optimization of the mist bioreactor for culture of hairy roots Biotechnol Bioeng 2010 107 802-813 (C) 2010 Wiley Periodicals, Inc