Context : The isocyanic acid ( HNCO ) presents an extended distribution in the centers of the Milky Way and the spiral galaxy IC342 .
Based on the morphology of the emission and the HNCO abundance with respect to H _ { 2 } , several authors made the hypothesis that HNCO could be a good tracer of interstellar shocks .

Aims : Here we test this hypothesis by observing a well-known Galactic source where the chemistry is dominated by shocks .

Methods : We have observed several transitions of HNCO towards L1157-mm and two positions ( B1 and B2 ) in the blue lobe of the molecular outflow .

Results : The HNCO line profiles exhibit the same characteristics of other well-known shock tracers like CH _ { 3 } OH , H _ { 2 } CO , SO or SO _ { 2 } .
HNCO , together with SO _ { 2 } and OCS , are the only three molecules detected so far whose emission is much more intense in B2 than in B1 , making these species valuable probes of chemical differences along the outflow .
The HNCO abundance with respect to H _ { 2 } is 0.4-1.8 10 ^ { -8 } in B1 and 0.3-1 10 ^ { -7 } in B2 .
These abundances are the highest ever measured , and imply an increment with respect to L1157-mm of a factor up to 83 , demonstrating that this molecule is actually a good shock tracer .

Conclusions : Our results probe that shocks can actually produce the HNCO abundance measured in galactic nuclei and even higher ones .
We propose that the gas phase abundance of HNCO is due both to grain mantles erosion by the shock waves and by neutral-neutral reactions in gas phase involving CN and O _ { 2 } .
The observed anticorrelation of CN and HNCO fluxes supports this scenario .
The observed similarities of the HNCO emission and the sulfured molecules may arise due to formation pathways involving also O _ { 2 } .