We report on the detection and first identification of the deuterium Balmer lines dα and dβ, observed in emission in the Orion Nebula (M 42). The excitation mechanism is UV fluorescence from the Lyman(D i) lines at the interface between the H ii region and the molecular cloud. These lines may open the possibility to measure D/H in galactic H ii regions and, e.g., low-metallicity extragalactic H ii regions, using optical spectroscopy. Fluorescence provides an extremely sensitive way to detect deuterium. Thus, the non-detection of dα and dβ in the planetary nebula NGC 6572 leads to the stringent upper limit (D/H)ngc 6572 less than the order of 1 × 10−7.