The purpose of this work was to identify the function of an open reading frame called YBL042, found during the systematic sequencing of Saccharomyces cerevisiae's chromosome II. The YBL042 gene product shows 70% similarity with the uracil permease and the allantoin permease encoded by FUR4 and DAL4, respectively. The mutation constructed by disruption of this ORF is allelic to the FUI1 gene previously described as encoding the uridine permease but not cloned yet. A strain carrying the disrupted allele and a fui1 mutant exhibit the same phenotype as they do not grow on a medium containing uridine as the sole source of pyrimidines and as they are resistant to 10(-3) M 5-fluorouridine (5FUI), a toxic analog of uridine. Even though the FUI1 gene has a multicopy suppressor effect on uracil transport, its product does not seem to be involved in this transport, in contrast to the FUR4 gene product which is involved in uridine transport. Moreover, the FUI1 gene product does not play any role in allantoin transport.