An acidophilic heterotrophic bacterium, Acidocella facilis sp. AIU409 produces an extracellular acid esterase when grown in a medium containing sorbitan mono ester, Tween 80. The estA gene encoding for the acid esterase was isolated from the genomic library of A. facilis AIU409, cloned into Escherichia coli MV1184, and the nucleotide sequence of the gene was determined. The structural gene of estA was found to be 1881bp. The open reading frame of estA encodes 627 amino acid residues (calculated molecular mass, 64,140 daltons). A rho-independent terminator was present just downstream of the termination codon, TGA. The deduced N-terminal amino acid showed that the presursor of the acid esterase had a signal peptide composed of 23 amino acids and a consensus sequence of lipase, G-X-S-X-S. The molecular mass excluding the signal peptide calculated from the deduced amino acid sequence if the acid esterase is 61,846. This is lower than the molecular mass, 64kDa estimated by gel electrophoresis. The predicated amino acid sequence of the acid esterase has high similarity to the acyltransferase from Aeromonas hydrophila and the lipase from Xenorhadbus luminescens.