In the context of the research project on the prehistoric settlement that we are carrying out in the Western coastline of Algeria, a number of excavations lead us to discover for the first time an important Acheulean site in the locality of Errayah near Mostaganem, a few kilometers from the Mediterranean Sea. The presence of the Acheulean culture in the coastal regions constitutes a particular scientific interest in learning more about the expansion of this culture in North Africa, and in studying the adaptation of the Acheulean group to the Mediterranean ecology. The archeological surveys and the systematic excavations carried out since 2001 have brought to light a continuous sequence of Acheulean stratigraphy at Errayeh, comprising four distinct archaeological levels. The preliminary data authenticate that this site represents the oldest Acheulean occupation in the Mediterranean coastline of Algeria. The excavations have revealed a rich and diversified lithic material made up of not only diagnosis elements of the Acheulean like for instance hand axes, cleavers, trihedral picks and large fragments, but also nuclei, varied fragments and touched up pieces. The preliminary study of the lithic Acheulean assembly pieces collected from the various levels of occupation are evidence of a great technological variability. The characterization of this variability is based on the production systems for each archaeological level. The analysis of the morphological and technical characters seeks primarily at defining the techniques of cutting up and shaping of the bifacial and non-bifacial tools. This approach also takes into account the diversity and the nature of materials used by the Acheuleans.