Heterodyne observations of Neptune have provided a measurement of the CO(2–1) line profile with a total bandpass of almost 8 GHz and a resolution of 4 MHz. The lineshape indicates that the CO mole fraction in Neptune's atmosphere is not uniform, but increases by a factor of ~2 from the troposphere/lower stratosphere (0.5 ppm at $p>20$ mbar) to the upper stratosphere (1 ppm at $p<20$ mbar). This indicates the existence of both external and internal sources of CO. The equivalent flux associated with the external source is ~$1\times10^8$ cm-2 s-1. We propose that the stratospheric CO results from a large (2 km) cometary impact that occurred ~200 years ago, although there remains problems with this hypothesis.