AbstractPrevious attempts to isolate autonomously replicating sequences (ars) from the dimorphic yeastYarrowia lipolyticahave been unsuccessful. We isolated a Fil−mutant unable to produce hyphae and growing only in a yeast form to facilitatearsisolation. This mutant was transformed with aY. lipolyticaDNA bank and several unstable clones were obtained. Extrachromosomal plasmids were evidenced in yeast recovered inEscherichia coliand characterized by restriction mapping. They were able to retransform Fil−and Fil+yeast strains at high frequency and transformants displayed a slightly unstable phenotype. The detailed analysis of the plasmids showed that only two differentarssequences had been isolated, each of them corresponding to a unique sequence in theY. lipolyticagenome. We concluded that functionalarssequences that can be cloned on plasmids are rare in this ye