Within the 80 kb aflatoxin pathway gene cluster characterized earlier, and between adhA and norA genes, we have identified an estA gene encoding an esterase from wild type strain Aspergillus parasiticus SRRC 143. The 1,500 bp genomic DNA and 945 bp cDNA sequences were determined for estA. Outside of the aflatoxin pathway gene cluster, an additional copy of the estA gene (named estA2) was also cloned from the same A. parasiticus strain. Comparison of the estA and estA2 sequences showed 9 substitutions within the 314 amino acid residues of their gene products, and no apparent defect was identified in the estA2. The estA gene is a homolog of the stcI gene identified in A. nidulans involved in the biosynthesis of sterigmatocystin and dihydro-sterigmatocystin for the conversion of versiconal hemiacetal acetate to versiconal. Reverse-transcriptase polymerase chain reaction (RT-PCR) experiments demonstrated that the estA is constitutively expressed. And only this estA gene, which is located within the aflatoxin pathway gene cluster, is expressed; no expression of the estA2 gene was detected under both aflatoxin conducive and non-conducive conditions. Possible reasons for the preferential expression of the estA over the estA2 gene have been discussed.