The gene encoding arylsulfatase (Ars; EC 3.1.6.1) as well as two Ars pseudogenes were isolated from sea urchin genomic libraries. The Ars gene was 20‐kbp long and contained six exons interrupted by five introns. Four polypyrimidine repetitive sequences were dispersed in its upstream‐flanking region.Comparison of the amino acid sequence of sea‐urchin arylsulfatase with those of human sterol sulfatase, human arylsulfatase A and bacterial arylsulfatase revealed that they have two similar sequences in common.The position of the transcription‐start site of the Ars gene was determined to be approximately 40‐bp upstream from the 5′ end of the protein‐coding region, and the nucleotide sequence of the 5′‐flanking region was determined up to 3.3 kbp upstream from the transcription start point. Putative TATAA box and CCAAT consensus sequences were located at positions ‐28 and ‐82, respectively. A highly conserved hexamer motif, CTCTTT, localized near the transcription‐start site of the sea‐urchin Ars gene, was also detected in similar regions of other sea urchin genes such as CyIIIa, Spec 1, Spec 2a, Spec 2c, Spec 2d, and SM50, but not in the histone genes.