Yeast dimorphism is an attractive model for the study of cell morphogenesis and differentiation. The non-conventional yeast Yarrowia lipolytica was chosen to characterise the regulation of dimorphic transition by extracellular pH and by the presence of organic sources of nitrogen. Organic nitrogen sources appear to be required for the morphogenic effect of pH. Two sets of mutants defective in either pH-dependent or nitrogen source-dependent signalling pathway were analysed. The results suggest that the latter but not the former is required for both normal filament formation on solid medium and pH-dependent dimorphic behaviour of Y. lipolytica in liquid medium. We propose that in this organism pH affects the formation of hyphae indirectly by modulation of availability and/or utilisation of transportable sources of nitrogen.