Based on observations carried out with the IRAM 30 m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).-- Accepted for publication in Astronomy & Astrophysics Letters.We present the results of a search for the negative ion C4H- in various dark clouds, low mass star-forming regions and photon-dominated regions (PDRs). We have also searched for C6H-, C2H- and CN- in some of the sources. The millimeter-wave observations were carried out with the IRAM-30m telescope. We detect C4H-, through the J = 9-8 and J = 10-9 rotational transitions, in the low mass star-forming region L1527. We thus confirm the tentative detection of the J = 9-8 line recently reported toward this source. The [C4H-]/[C4H] ratio found is 0.011 %, which is slightly lower than the value observed in IRC +10216, 0.024 %, but above the 3 sigma upper limit we derive in TMC-1, < 0.0052 %. We have also derived an upper limit for the [C6H-]/[C6H] ratio in the Horsehead Nebula, and for various anion-to-neutral ratios in the observed sources. These results are compared with recent chemical models.This work has been supported by Spanish MEC trough grants AYA2003-2785, AYA2006-14876 and ESP2004-665 and by Spanish CAM under PRICIT project S-0505/ESP-0237 (ASTROCAM). MA also acknowledges grant AP2003-4619 from Spanish MEC.SI