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 and 70. However, HCNO was not detected toward the direction of the cyanopolyyne peak of TMC-1 or toward 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 CH₂ + NO --> HCNO + H is a key reaction for the formation of fulminic acid. A value of 5.5 × 10^-12 cm³ s^-1 of the corresponding reaction rate coefficient, as given by Miller et al., allows us 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.