The microwave spectra of ten isotopic species of trans-propargyl amine have been measured in the frequency range 8000-40000 MHz. The values of the rotational constants obtained from the measured transitions for these isotopic species were used in a diagnostic least-squares calculations yielding an r0 structure. A partial rs structure was also obtained. The <HCH angle is considerably smaller and the <NCC angle considerably larger than the tetrahedral value. The dipole moments μa and μb have been determined. No experimental evidence has been found for the gauche conformer in the microwave region. Considerations on the relative intensity of the rotational lines of the normal species compared with those of the isotopic C 13 and 15N species in natural abundance allow an estimation of a lower limit for the energy difference between the two conformers.