The first detection is reported of both the nitrogen fifteen isotopes HN-15N(+) and HNN-15(+) in an astronomical source, the molecular cloud associated with DR 21(OH), and the finding is used to estimate the relative abundance of HN2(+). Stringent limits on the abundance of these isotopic species are also derived in several molecular clouds. The two isotopic species have nearly equal abundances as predicted by ion molecule formation of HN2(+). Comparison with the weakest component of HN2 (F = 1-0) yields an intensity ratio N-14/N-15 over 150, consistent with the terrestrial ratio. The N2H(+)/HCO(+) abundance ratio is found to be about 0.9. This value suggests that in DR 21(OH), nitrogen could be mostly in the form of N2 in the gas phase, while only 10 to 15 percent of carbon is in the form of gaseous CO. In four other molecular clouds, only upper limits could be established for the N15 emission.