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The cumulene carbenes are important components of hydrocarbon chemistry in low-mass star-forming cores. Here we report the first astronomical detection of the long-chain cumulene carbene H 2 C 6 in the interstellar cloud TMC-1, from observations of two of its rotational transitions: J K,KЈ ϭ 7 1,7 3 6 1,6 at 18.8 GHz and 8 1,8 3 7 1,7 at 21.5 GHz, using NASA's Deep Space Network 70 m antenna at Goldstone, California. In addition we also observed the shorter cumulene carbene H 2 C 4 at the same position. The fractional abundance of H 2 C 6 relative to H 2 is about 4.7 ϫ 10 Ϫ11 and that of H 2 C 4 is about 1.1 ϫ 10 Ϫ9. The abundance of H 2 C 6 is in fairly good agreement with gas-phase chemical models for young molecular cloud cores, but the abundance of H 2 C 4 is significantly larger than predicted.

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