: For the first time, collective bands have been observed in the doubly-odd nucleus 124Cs. They are proposed to result in the coupling of an hll/2 proton to h11/2 and positive parity neutrons. The relative residual protonneutron interaction in 124Cs is calculated and compared to that of neighbouring odd-odd nuclei.The knowledge of doubly-odd nuclei in the Xe-Ce transitional region is limited, especially that of the light ones. However there are data, for example those recent for 128,130La [1] and 126La [2], which show that two quasiparticle collective bands excited in heavy-ion induced reactions are generated by the coupling of h11/2 proton to hl 1/2 and g7/2 (and/or d5/2) neutrons. This region is well suited to study the interplay between the neutron and the proton which are situated in the same shell and produce opposite y-driving effects. The study of odd-odd nuclei can bring new information relative to their coupling and in particular how these opposite "?-driving forces compensate each other. We present data for two collective bands observed in 124Cs. Preliminary results have already been reported [3].Many low-energy low-spin states are already known in 124Cs from previous measurements. Their energies, spins and parities have been determined by studying the decay of on -line mass separated samples of 124mCs [4] and 124Ba [5]. Two isomers (7 +, T1/2 = 6.3 s and 4-, T1/2 = 69 ns) have been identified.The experiments were performed at the heavy-ion SARA accelerator at Grenoble by using the ll5In(12C, 3n) reaction at 56 MeV beam energy. The y-rays were detected by one planar Ge detector and two coaxial detectors. Coincidence data were recorded in a conventional 4-parameter mode (ET1, E72, ty1 -RF, tT1 -ty2). Angular distributions have been measured at 4 angles.Two groups of levels corresponding to two different collective excitations are discussed. They are characterized by sets of M1 transitions and E2 cross-overs and different energy splittings as shown in Fig. 1. The energies of the M1 transitions are such that the separation between both signature branches is about 4 or 5 times larger for band 1 than for band 2. This situation for 124Cs is comparable to that observed for its isotone 126La [2]. The two bands presented in Fig. 2 have very likely the same configurations as those proposed for the doubly-odd nuclei 126-130La. As for the other odd-odd nuclei in this mass region, bandhead spin and parity of these two collective bands in 124Cs are unknown so far. The parities indicated in Fig.2 are deduced from the products of the parities of the single particle orbitals and the spins are only tentative. However, because of the combinations of the Routhians of the single particles considered, we can be confident of their even and odd character i.e. the lowest levels in bands 1 and 2 have even and odd spin, respectively. The Gallagher-Moskowski coupling rules predict quantum numbers K = 4 and 3 for the :~hll/2 | Vhll/2 and ~hll]2 | v(~ = +) configurations. The two bands are not positioned in the level scheme...