Chlorophyll biosynthesis requires a metabolic dialog between the chloroplast envelope and thylakoids where biosynthetic activities are localized. Here, we report the first plant S-adenosyl-l-methionine:Mg-protoporphyrin IX methyltransferase (MgP(IX)MT) sequence identified in the Arabidopsis genome owing to its similarity with the Synechocystis sp. MgP(IX)MT gene. After expression in Escherichia coli, the recombinant Arabidopsis thaliana cDNA was shown to encode a protein having MgP(IX)MT activity. The full-length polypeptide exhibits a chloroplast transit peptide that is processed during import into the chloroplast. The mature protein contains two functional regions. The C-terminal part aligns with the Synechocystis full-length protein. The corresponding truncated region binds to Ado-met, as assayed by UV crosslinking, and is shown to harbor the MgP(IX)MT activity. Downstream of the cleaved transit peptide, the 40 N-terminal amino acids of the mature protein are very hydrophobic and enhance the association of the protein with the membrane. In A. thaliana and spinach, the MgP(IX)MT protein has a dual localization in chloroplast envelope membranes as well as in thylakoids. The protein is active in each membrane and has the same apparent size corresponding to the processed mature protein. The protein is very likely a monotopic membrane protein embedded within one leaflet of the membrane as indicated by ionic and alkaline extraction of each membrane. The rationale for a dual localization of the protein in the chloroplast is discussed.