AbstractA multi‐stage pilot fermentor was designed which consists of nine stainless steel cylindrical compartments 2 ft. in length and 8 in. in diameter, joined end to end, giving a tube with a total length of 18 ft. The whole apparatus was mounted with its axis in a horizontal plane. Each compartment was sealed off from its neighboring compartment by a separatory plate having a 1‐in. overflow hole in the upper half. Shafts 9½ feet long extended from each end of the fermentor through the compartments to the center. Agitation was provided by a multitude of stainless steel blades mounted at right angles to the shafts and spaced at regular intervals along the shafts. Stationary, stainless steel comb‐shaped baffle plates were installed vertically at the bottom of each compartment to increase mixing efficiency. Constant leaving and re‐entering of the fermentation liquor by the blades upon rotation imparts a shattering of the liquid surface thus preventing the accumulation of mycelial aggregates on the walls above the fermentation liquor. The mechanical performance of the described fermentor has proved excellent and several trial fermentations (antibiotic formation and steroid bioconversion) are d