We present the discovery of shock-excited NH 3 in a well-collimated jet associated with the extremely young high-mass star IRAS 20126ϩ4104. The NH 3 (3, 3) and (4, 4) emission is dominated by three clumps along the SiO jet. At the end of the jet, there exists strong and broad ‫01ע(‬ km s ) NH 3 (3, 3) emission. With typical Ϫ1 brightness temperatures greater than 500 K, the overall emission indicates a weakly inverted population and appears in an arc, consistent with the excitation by bow

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... . There are two bright spots in the NH 3 (3, 3) emission with brightness temperatures of approximately 2000 K. The narrow line width (1.5 km s FWHM), Ϫ1 the small sizes (!0Љ .3), and the unusually high brightness temperature of the features are indicative of maser emission. Our observations provide clear evidence that NH 3 (3, 3) masers are excited in shock regions in molecular outflows.