A novel post-translational protein modification has recently been described in two human sulfatases, by which a cysteine is replaced by a serinesemialdehyde (2-amino-3-oxopropionic acid) residue [Schmidt, B., Selmer, T., Ingendoh, A. & von Figura, K. (1995) Cell 82, 271-278]. This cysteine is conserved among all known eukaryotic sulfatases. Here we report the presence of this modification in arylsulfatase from the green alga Volvox carteri. The evolutionary conservation of this novel protein modification between sulfatases of V. carteri and man lends further support to the assumption that this modification is required for the catalytic activity of sulfatases and may be present in all sulfatases of eukaryotic origin.