The effects of nine organic extractive reagents on growth and acid production by Clostridium butyricum S21 were examined to evaluate their possible use in extractive fermentation of butyric acid. Oleylalcohol alone and in a mixture with Hostarex A327 (n-octyl(n-decyl)amine) and pure tri-hexylphosphate (THP) did not appear to be toxic. The presence of THP decreased the selectivity of butyrate production. High toxicity was shown by n-octanol and isodecanol alone and in mixtures with the extractant Hostarex A327 and diluent n-alcanes. An acceptable decrease of butyric acid concentration, productivity and production selectivity (7.26, 0.33 and 0.54, respectively) compared with optimal pH of 6.5 (11.75, 0.60 and 0.73, respectively) was obtained at pH 5.2. At this pH the distribution coefficient of butyric acid between water and Hostarex A327 (20% w/w) in oleylalcohol was about 3.0. It was four times lower than for the diluted solution (13.2) and two times lower than for the concentrated (6.5) solution of butyric acid at pH 4.0, but about three times higher than in the case of pure oleylalcohol (1.01) at pH 5.2. Hostarex A327 (20% w/w) in oleylalcohol as an extractive reagent at a pH of 5.2, can be used for the production and in situ extraction of butyric acid.