Zinc ion homeostasis in Saccharomyces cerevisiae is controlled primarily through the transcriptional regulation of zinc uptake systems in response to intracellular zinc levels. A high-affinity uptake system is encoded by the ZRT1 gene, and its expression is induced more than 30-fold in zinc-limited cells. A low-affinity transporter is encoded by the ZRT2 gene, and this system is also regulated by zinc. We used a genetic approach to isolate mutants whose ZRT1 expression is no longer repressed in zinc-replete cells, and a new gene, ZAP1, was identified. ZAP1 encodes a 93-kDa protein with sequence similarity to transcriptional activators; the C-terminal 174 amino acids contains five C2H2 zinc finger domains, and the N terminus (residues 1 to 706) has two potential acidic activation domains. The N-terminal region also contains 12% histidine and cysteine residues. The mutant allele isolated, ZAP1-1up, is semidominant and caused high-level expression of ZRT1 and ZRT2 in both zinc-limited and zinc-replete cells. This phenotype is the result of a mutation that substitutes a serine for a cysteine residue in the N-terminal region. A zap1 deletion mutant grew well on zinc-replete media but poorly on zinc-limiting media. This mutant had low-level ZRT1 and ZRT2 expression in zinc-limited as well as zinc-replete cells. These data indicate that Zap1p plays a central role in zinc ion homeostasis by regulating transcription of the zinc uptake system genes in response to zinc. Finally, we present evidence that Zap1p regulates transcription of its own promoter in response to zinc through a positive autoregulatory mechanism.