Comets are believed to have formed at the same time as the other bodies in the Solar System, but at a sufficient distance from the proto-Sun that their composition was unaffected by fractionation. Thus, they may preserve intact the chemical composition of the original solar nebula, and perhaps of much of the Galaxy. Previously we demonstrated1,2 that one component of the interstellar dust grains has an organic composition, by detecting a broad absorption band at a central wavelength of 3.4 μm along the line of sight to IRS7, a luminous star near the galactic centre. We have performed the same observations of comet Halley, and now report the detection of a similar emission band in the comet's spectrum. This evidence of organic grains in comets indicates the possible presence therein of a component of similar composition to that found in micrometeorites. Although no definite identification is possible we note that a feature near 3.4 μm is predicted by various models of interstellar dust3–7.