As far as we know, only one example (1) has been reported up to now of differential negative resistance (S-type) in GaP. This effect was obtained on alloyed p.n junctions, with p-type Zn and Sn doped crystals, and it was attributed to a Lampert's (~) doubleinjection mechanism. We report here on differential negative-resistance effects obtained, for the first time, in samples without junctions, and with simple rectifying In contacts, at 77 °K. The GaP samples used in our experiment (solution-grown single crystals) were essentially of two types: the first one, with (10 a-104) ~2 cm resistivity and (1018-101~) cm -a impurity content, the latter one, more heavily doped, with (10--102) g2 cm resistivity and _TV~--tV~ in the range (101s--10 l~) cm -s. The doping was made with N (neutral donor), S (E~ ~ 107 meV) and C (E~ ~ 48 meV). The samples (typical dimensions (0.5×0.1)<0.1) cm a) were mounted with In-soldered electrodes, showing a rectifying diode behaviour, and were immersed in liquid nitrogen. The I-V characteristic was obtained with a 575 Tektronics curve tracer, which gives a rectified alternate voltage (200 V maximum peak, 100 Hz repetition rate). Contemporarily, the intense radiative recombination emission present in all the samples has been detected. The spectrum showed a discrete structure typical in GaP, with a series of peaks in the region from 5300 to 5800 ~, due to bound-exciton transition and to donor-acceptor (S-C) pair recombinations. The detailed results and their connection to the negative resistance and to the photoluminescence will be discussed in a further paper. Essentially, two types of negative resistance effects were found. The first one, (Fig. la) and b)), appearing at relatively low current levels (of the order of 5 mA_), was present only in the first type, less heavily doped, of GaP crystals. This effect, of the S-type, was characterized by the contemporary presence of oscillations, as one can see in Fig. la), where a detail of a typical forward I-V curve is shown. The oscillations start at a threshold between 100 and 300 V/cm; they have a fre~ quency of the order of ~ 200 kHz and it is also possible to excite them with a D.C. voltage supply (Fig. 2). At higher current levels, further phenomena of current insta