<jats:p> Siderophores are extracellular iron-binding compounds that mediate iron transport into many cells. We present evidence of analogous molecules for copper transport from methane-oxidizing bacteria, represented here by a small fluorescent chromopeptide (C <jats:sub>45</jats:sub> N <jats:sub>12</jats:sub> O <jats:sub>14</jats:sub> H <jats:sub>62</jats:sub> Cu, 1216 daltons) produced by <jats:italic>Methylosinus trichosporium</jats:italic> OB3b. The crystal structure of this compound, methanobactin, was resolved to 1.15 angstroms. It is composed of a tetrapeptide, a tripeptide, and several unusual moieties, including two 4-thionyl-5-hydroxy-imidazole chromophores that coordinate the copper, a pyrrolidine that confers a bend in the overall chain, and an amino <jats:italic>-</jats:italic> terminal isopropylester group. The copper coordination environment includes a dual nitrogen- and sulfur-donating system derived from the thionyl imidazolate moieties. Structural elucidation of this molecule has broad implications in terms of organo-copper chemistry, biological methane oxidation, and global carbon cycling.