The two most intense lines of the 18-cm spectrum of 17OH have been detected in absorption against Sgr A. Rest frequencies of |$1626\cdot17\pm0\cdot03\,\text{and}\,1624\cdot43\pm0\cdot03\,\text{MHz}$| have been derived. The number ratio |$^{18}\text{OH}/^{17}\text{OH}\,\text{of}\,4\cdot2\pm0\cdot4$| is less than the terrestrial value of 5·5. It is concluded that the 17O/16O abundance ratio near the galactic centre is greater than the terrestrial value.