The Onsala 20 m telescope, equipped with a travelling wave maser (frequency range ∼29-35 GHz; zenith system noise temperature ∼90-160 K) has been used in a search for HC5N (J=12-11 and 13-12) and HC7N (J = 28-27) to a low detection limit. In addition to observations of the Taurus dust clouds TMC1, TMC2 and L1544, the HC5N (J =13-12) transition has been detected at a low level (TA∗ ∼ 0.1 K; p-t-p noise ∼ 0.1 K for a velocity resolution of 0.04 km s-1) in the isolated globule B335. The total HC5N column density estimated from the very narrow line (Δv ∼ 0.2 km s-1) is 6·1011 cm-2. From our non-detection of HC5N in L134N a total column density ≲ 5·1011 cm-2 is deduced.