Ammonia towards the UCHII—region NGC 7538—IRS 1 shows an extremely hot component observed in absorption in several transitions and a so far unexplained (9,6) maser (see Fig. 1). This source is the only known source in the Galaxy where the 141 GHz line (1450 K above ground) of vibrationally excited Ammonia is seen in absorption (Schilke et al. 1990). An additional unique feature is the 15NH3(3,3) maser observed by Mauersberger et al. (1986) in Effelsberg and by Johnston et al. (1989) with the VLA. There are theoretical models to explain such a maser (see e.g. Flower, Offer and Schilke 1990) in low density regions where ortho-H2 is underabundant. In this study, our aim was to investigate 15NH3 in more detail. We succeeded in observing the (1,1) and (2,2) lines of 15NH3 in absorption and, surprisingly, the (4,3) and (4,4) lines in emission (see Fig. 1).