While the rotational spectrum of trans-HCOOH is well known, the cis-rotamer was known previously only from a few microwave transitions. The spectra of cis-and trans-HCOOH and of trans-H 13 COOH have been identified in the far infrared Fourier transform spectrum recorded in the range 20-100 cm -1 . The measurements were performed using an 80-cm base length White-type multipass absorption cell in combination with a Bruker IFS 120 HR spectrometer. The White cell was set up for 90 passes (total path length = 72 m) and the spectrum was measured with a resolution (FWHM) of 0.0012 cm -1 . Submillimeterwave transitions, just under 1 terahertz, of the same three species of formic acid were measured in the range from 835 to 993 GHz (27.8-33.1 cm -1 ) and it was possible to assign b-type Q-branch transitions of both cis-and trans-HCOOH in this region, and also transitions of trans-H 13 COOH. These measurements served furthermore as a check on the calibration of the FTIR measurement. Furthermore, new millimeter-wave measurements in the range 172-366 GHz were made, bridging the gap between the MW and FIR lines for cis-HCOOH and providing important data for trans-H 13 COOH. The assigned FTIR, submillimeter-and millimeterwave data reported in this work were fitted together with previously published rotational transitions to obtain rotational constants, using Watson's A-reduced Hamiltonian, for cis-HCOOH, trans-HCOOH, and trans-H 13 COOH.