The far-infrared spectrum of phosphine, PH 3, was recorded in the region between 30 and 200 cm -1 at a resolution of 0.002 cm -1. ∆ J = +1, ∆ K = 0 rotational transitions in the ground state were measured and assigned up to J″ = 22 and K = 19. These transitions were analyzed together with the presently available microwave and submillimeter-wave data on the basis of different formulations of the rotational Hamiltonian, which included ∆ k = ±3 and/or ∆ k = ±6 interaction terms. An upper limit for the constant of the inversion splitting was obtained by fitting the same transitions to an appropriate inversion-rotational Hamiltonian. Rotational transitions in the v2 = 1 and v4 = 1 vibrational states were also observed.