The observations of three rotational transitions (J = 70 → 69, 71 → 70, and 72 → 71) of HC11N (cyano-deca-penta-yne) in the microwave emission spectrum of the circumstellar envelope of the cool carbon star IRC + 10°216 are reported here. The abundances relative to molecular hydrogen and HC7N are estimated to be ∼7×10−8 and ∼0.7, respectively. With these observations, taken during March and May 1981 at the Haystack Observatory, HC11N becomes the largest and heaviest molecule yet detected outside the Earth's atmosphere.