The aim of the present study was to assess the potential of valorisation of a solid industrial derivative of tallow, composed of saturated free-fatty acids ("stearin"), by fermentations carried out by the yeast Yarrowia lipolytica ACA-DC 50109 in order to produce microbial lipid, biomass and extra-cellular lipase. High quantities of biomass were produced (biomass yield of around 1.1 ± 0.1 g of total biomass produced per g of fat consumed) when the organism was grown in shake flasks, regardless of the concentration of extra-cellular nitrogen present. Cellular lipids accumulated in notable quantities regardless of the nitrogen availability of the medium, though this process was clearly favoured at high initial fat and low initial nitrogen concentrations. The maximum quantity of fat produced was 7.9 mg/ml corresponding to 52.0% (wt/wt) of lipid in the dry biomass. Lipase production was critically affected by the medium composition and its concentration clearly increased with increasing concentrations of fat and extra-cellular nitrogen concentration reaching a maximum level of 2.50 IU/ml. Lipase concentration decreased in the stationary growth phase. In bioreactor trials, in which higher agitation and aeration conditions were employed compared with the equivalent trial in the flasks, significantly higher quantities of biomass were produced (maximum concentration 30.5 mg/ml, yield of 1.6 g of total biomass produced per g of fat consumed) while remarkably lower quantities of cellular lipids and extra-cellular lipase were synthesised. Numerical models successfully simulated both conversion of substrate fat into biomass and production and subsequent hydrolysis of extra-cellular lipase and presented a satisfactory predictive ability verifying the potential for single-cell protein and lipase production by Yarrowia lipolytica ACA-DC 50109. In all cultures, the mycelial form of the culture was dominant with few single cells present