Microwave emission from cyanoacetylene (HC,N) has been detected in the galactic radio source Sgr B2. Two of the three quadru ole-split transitions of the j = 0 H 1 transition were observed, the strongest with a strength of 4 .u. The emission is not linearly polarized. Within the frequency uncertainty of the transitions the HC,N lines in Sgr B2 have the same radial velocity, +62 km s ', as the formaldehyde absorption. No HC,N was detected in sizteen other galactic sources in which the molecules OH, H,CO, and HCN have been detected.