The production of the polyene antibiotic, pimaricin by the organism Streptomyces natalensis CBS 700.57 was studied using an apparatus comprising a 10 litre fermentation vessel, and facilities for temperature, pH, dissolved oxygen, vessel pressure and gas composition measurement. The apparatus was connected via a Texas Instruments PM550 programmable logic controller to an IBM-compatible computer in order to record data from the fermentation and to control certain parameters. Adjustment of medium composition, and the use of fed batch techniques did not produce any improvements in pimaricin yields. However, a crucial role for carbonate in stimulating the production of the antibiotic was demonstrated. Yields as high as 1 g/litre were obtained. Patterns in dissolved oxygen were observed which suggested that autolytic processes may occur towards the end of the fermentation due to glucose depletion. The production of the antibiotic was not shown to be dissociated from cell growth. An assay method, using High Performance Liquid Chromatography, for routinely estimating the levels of pimaricin in fermentation broths was successfully developed.