al. 1993), the \[SII/ZnII\] ratio is an ideal diagnostic for understanding the character of the chemical evolution. So far, only few determinations of \[S/Zn\] ratio in DLA are present in the literature. Meyer et al. (1989) found \[S/Zn\]=$-$0.1 in the DLA at $z=2.8$ towards QSO PKS 0528-250 and Green et al. (1995) found \[S/Zn\] = +0.1 in the $z$=1.775 DLA in Q1331. Thus, in spite of the very poor metallicity of the gas, which in these systems is $\simeq$ -2.0, the ratios of elements non depleted in dust have solar values.

In DLA systems the relative abundances of Si and Fe are often found to be consistent with a halo pattern. Lu et al. (1995a) found \[Si/Fe\]=+0.31 for the DLA at $z=2.844$ towards HS 1946=76 and Pettini et al. found \[Si/Fe\]=+0.4 for the DLA at $z=2.27$ towards QSO 2348-147. However, Si and Fe are differentially depleted from gas to dust in our Galaxy. The average value in the compilation of 11 clouds observed with HST made by Lu et al (1995a, their Table 8) is \[Si/Fe\]=+0.66$\pm0.26$. The observed enhancement of Si versus Fe in the DLA would reflect the overabundances of $\alpha$ elements with respect to the iron-peak elements only 