Abstract. Medium-spin, yrast excitations in the138 Cs nucleus, populated in the spontaneous fission of 248 Cm, were observed for the first time.138 Cs was studied by means of prompt γ-ray spectroscopy using the EUROGAM2 array. The newly observed yrast cascade, built on the known 6 − isomer at 80 keV, was successfully described by shell model calculations. (see refs. [3,5] for details). Similarly good description was also obtained in our study of an analogous 6 − isomer in the isotone 136 I [6] where the single-particle energy of the proton d 5/2 orbital had to be lowered by 400 keV relative to its position in 133 Sb. This puzzling effect was first observed in 135 Sb and interpreted as due to the formation of neutron skin past the N = 82 a e-mail: Teresa.Rzaca@fuw.edu.pl shell [7,8]. Using the SMPN set we confirmed the need to lower the d 5/2 orbital in 135 Sb but in ref.[6] we argued against the neutron skin hypothesis, noting that its formation should be a slow phenomenon like other changes in this region connected with the growing number of neutrons, whereas the need for a decrease of d 5/2 s.p. energy was rather abrupt. Instead, we suggested that the lowering of the πd 5/2 s.p. energy may be an artificial effect, which surprisingly well accounts for deficiencies of some of the tbme's, possibly proton-neutron interactions. To propose any improvement one would like to know how the effect varies with the proton and the neutron number. In fig. 1 we show the approximate shifts (in keV), which have to be applied to the πd 5/2 s.p. energy in various nuclei in the 132 Sn region in order to reproduce them properly. Figure 1 suggests that the effect grows with the number of valence neutrons and/or the number of πν pairs. It is thus of interest to find its magnitude in the 138 Cs nucleus. On one hand, fig. 1 suggests a non-zero value, on the other hand, the calculations of ref.[3] done