Carbon monoxide has been detected in the interstellar absorption spectrum of Ophiuchi. Equivalent widths are presented for eight transitions in the (A i fourth positive system of Ci and for four of 13C160. A satisfactory fit to a curve of growth can be obtained only when the rotational states are assumed populated by the universal blackbody radiation. A column density of 7.6 X 1015 molecules cm for 12C16O and of 0.072 X 1015 molecules cm for 5'C' is found, which yields a 12C/13C ratio of 105. The column density of atomic hydrogen is 4.6 X 1O atoms cm , in agreement with the value previously reported by one of us.