Sequence analysis of the arginase/agmatine ureohydrolase family, important enzymes in arginine/agmatine metabolism and the urea cycle, reveals the similarity of arginases to formiminoglutamate hydrolase (hutG) in Klebsiella aerogenes and to a previously unidentified open reading frame adjacent to the HMf locus of the archaebacterium Methanothermus fervidus. The gene structure and distribution of these homologous proteins across primary kingdoms suggest that this family is another example of a primordial enzyme possibly present in the universal common ancestor and that can be used as phylogenetic marker.