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In response to excess metal, higher plants produce metal-binding peptides ([γEC]_nG) whose biosynthesis is believed to be mediated by enzymes involved in glutathione (γECG) metabolism. In contrast, animals synthesize metallothioneins, gene-encoded low molecular weight cysteine-rich metal-binding proteins. In an investigation of copper-regulated genes in the copper-tolerant flowering plant Mimulus guttatus, we have isolated a series of cDNA clones identifying two genes which encode a protein with class I metallothionein domains. This represents the first description of a metallothionein gene in a flowering plant.