Abstract. Using the atomic beam magnetic resonance method, the five hyperfine structure separations in the 4f 3 6s 2 419. 2 ground state of 141 _ 59P1 have been measured. The results are:6477.913423 (17) 6 5 5556.359848 (6) 5 4 4633.023306 (2) 4 3 3708.201146 (5) 3 2 2782.190601 (15) From these quantities, the multipole interaction constants Ak, k = 1, 2, 3, 4 between the nucleus and the electron shell have been calculated. A 4 especially then served to give the following limit for the intrinsic hexadecapole moment: IQ4ol <0.4eb 2. Furthermore, the gs-factors of the 41 multiplet have been measured at magnetic fields of 300 Oe. The results are: gj(419.2) =0.731 037 1(15) gj(4Ii1~2) =0.965 1476 (20) gj(41132) = 1.106 319 7(40) gj(d'I152)= 1.197963 (30) Small corrections due to perturbations by neighbouring fine structure levels are included.