<title>Abstract</title>Aquatic heavy metal pollution is a growing concern. To facilitate heavy metal monitoring in water, we developed transgenic <italic>Daphnia</italic> that are highly sensitive to heavy metals and respond to them rapidly. Metallothionein A, which was a metal response gene, and its promoter region was obtained from <italic>Daphnia magna</italic>. A chimeric gene fusing the promoter region with a green fluorescent protein (GFP) gene was integrated into <italic>D. magna</italic> using the TALEN technique and transgenic <italic>Daphnia</italic> named <italic>D. magna</italic> MetalloG were produced. When <italic>D. magna</italic> MetalloG was exposed to heavy metal solutions for 1 h, GFP expression was induced only in their midgut and hepatopancreas. The lowest concentrations of heavy metals that activated GFP expression were 1.2 µM Zn²⁺, 130 nM Cu²⁺, and 70 nM Cd²⁺. Heavy metal exposure for 24 h could lower the thresholds even further. <italic>D. magna</italic> MetalloG facilitates aqueous heavy metal detection and might enhance water quality monitoring.