Context : Precise S abundances are important in the study of the early chemical evolution of the Galaxy .
In particular the site of the formation remains uncertain because , at low metallicity , the trend of this \alpha -element versus [ Fe/H ] remains unclear .
Moreover , although sulfur is not bound significantly in dust grains in the ISM , it seems to behave differently in DLAs and old metal-poor stars .

Aims : We attempt a precise measurement of the S abundance in a sample of extremely metal-poor stars observed with the ESO VLT equipped with UVES , taking into account NLTE and 3D effects .

Methods : The NLTE profiles of the lines of multiplet 1 of S I were computed with a version of the program MULTI , including opacity sources from ATLAS9 and based on a new model atom for S. These profiles were fitted to the observed spectra .

Results : We find that sulfur in EMP stars behaves like the other \alpha -elements , with [ S/Fe ] remaining approximately constant below [ Fe/H ] =–3 .
However , [ S/Mg ] seems to decrease slightly with increasing [ Mg/H ] .
The overall abundance patterns of O , Na , Mg , Al , S , and K are most closely matched by the SN model yields by Heger & Woosley .
The [ S/Zn ] ratio in EMP stars is solar , as also found in DLAs .
We derive an upper limit to the sulfur abundance [ S/Fe ] < +0.5 for the ultra metal-poor star CS 22949-037 .
This , along with a previously reported measurement of zinc , argues against the conjecture that the light-element abundance pattern of this star ( and by analogy , the hyper iron-poor stars HE 0107-5240 and HE 1327-2326 ) would be due to dust depletion .

Conclusions :