Twenty-seven Yarrowia lipolytica strains as well as five strains from three other species of the Yarrowia clade (Aciculoconidium aculeatunt, Candida hispaniensis and Candida bentonensis) were screened for citric acid production with pure glycerol as the carbon source. The cultures were grown under nitrogen-limited conditions. None of the non-Yarrowia strains produced citric acid, although they were able to grow on glycerol. All of the Y lipolytica strains were able to produce citric acid in varying concentrations and, under the screening conditions used, the yields obtained formed a continuum from very low to among the highest reported for this substrate. The highest yielding strain, Y lipolytica NRRL YB-423, produced 21.6 g/L citric acid from 40 g[L glycerol (54% yield). The citric acid to isocitric acid ratio produced by this strain in the initial screen was 11.3, while most of the strains produced ratios of between 2 and 6. Further work on medium optimization with this strain showed that the optimum carbon to nitrogen ratio for the rate of citric acid production was 172 while the best combination of rate and yield was obtained at a C/N ratio of 343. The citric acid to isocitric acid ratios produced reached an optimum at C/N ratios of 343-686. Published by Elsevier Inc.