Context :

Aims : We present the results of a search for the negative ion C _ { 4 } H ^ { - } in various dark clouds , low mass star-forming regions and photon-dominated regions ( PDRs ) .
We have also searched for C _ { 6 } H ^ { - } , C _ { 2 } H ^ { - } and CN ^ { - } in some of the sources .

Methods : The millimeter-wave observations were carried out with the IRAM-30m telescope .

Results : We detect C _ { 4 } H ^ { - } , 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 [ C _ { 4 } H ^ { - } ] / [ C _ { 4 } H ] 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 [ C _ { 6 } H ^ { - } ] / [ C _ { 6 } H ] ratio in the Horsehead Nebula , and for various anion-to-neutral ratios in the observed sources .
These results are compared with recent chemical models .

Conclusions :