The ability of 10 yeasts, isolated from the waste treatment system at a commercial rendering operation, to utilise tallow (20 g L"1) as a sole carbon source was investigated. One isolate, identified as Yarrowia lipolytica , demonstrated superior fat removal ability and consequently was chosen for further studies in the development of a microbial-based fat removal system. Initially, the influence of temperature, medium pH, agitation and nitrogen source was assessed in shake flask studies. Maximum tallow removal of 75% in 168 hours was achieved under the optimal conditions of 25°C, pH 7.0, 130 rpm and 0.4 g N L'1, irrespective of whether ammonium sulphate, urea or peptone was employed as the nitrogen source. Medium pH was controlled using 0.1 M potassium phosphate buffer. Addition of glucose (1.0 and 10.0 g L'1) to the medium under optimal environmental conditions did not inhibit tallow removal and there was a concomitant use of both substrates. Acclimation of the yeast inoculum to lipids did not influence fat removal. The fat removal system was scaled-up to 2 L and 10 L fermentation. Under optimum aeration of 1 VVM and agitation of 500 rpm and 1000 rpm, the time required to achieve maximum tallow removal was significantly reduced from 168 to 65 hours in the 2 L and 10 L fermenters, respectively. Potassium was key to optimal fat removal with the requirement of greater than 60 mM K+ in the medium by the yeast. Cellular potassium levels of 80 nmol K+ (106 cell) ' 1 corresponded with maximum growth and fat removal. Extracellular biosurfactant production was detected under optimal growth conditions, which corresponded with emulsification of the tallow in the growth vessel. Y. lipolytica is a dimorphic yeast however, mycelial growth was not considered to play an important role in fat removal by the yeast.