  We report on the first detection in space of fulminic acid, HCNO. This isomer
of HNCO has been observed in three starless cores, B1, L1544 and L183, and in
the low mass star forming region L1527 with a measured abundance ratio of
HNCO/HCNO between 40-70. However, HCNO was not detected towards the direction
of the cyanopolyyne peak of TMC-1 or towards the Orion Hot Core region. The
derived HNCO/HCNO abundance ratio in these cases is greater than 350 and 1000
in TMC-1 and Orion, respectively. We find that CH_2 + NO \to HCNO + H is a key
reaction for the formation of fulminic acid. A value of 5.5 10^{-12} cm^3
s^{-1} of the corresponding reaction rate coefficient, as given by Miller et
al. (2003), allows to reproduce the observed abundances of fulminic acid in
both the observed dark clouds and low mass star forming core, where the
determined abundance of HNCO in these regions with respect to molecular
hydrogen is 1-5 10^{-10}.
