map = new OpenLayers.Map('map', {controls: [], displayProjection: new OpenLayers.Projection("EPSG:32632"), numZoomLevels: 31, tileSize: new OpenLayers.Size(256,256)});
wms1 = new OpenLayers.Layer.WMS('base', 'http://172.26.10.126/fcgi-bin/mapserv.exe?map=/ms4w/apps/guinea/guinea.map', {layers: 'base'},{isBaseLayer:true, visibility:true,transparent:true});
wms2 = new OpenLayers.Layer.WMS('guinea','http://172.26.10.126/fcgi-bin/mapserv.exe?map=/ms4w/apps/guinea/guinea.map', {layers: 'guinea'},{isBaseLayer:false, visibility:true,transparent:true});
wms3 = new OpenLayers.Layer.WMS('municipios','http://172.26.10.126/fcgi-bin/mapserv.exe?map=/ms4w/apps/guinea/guinea.map', {layers: 'municipios'},{isBaseLayer:false, visibility:false,transparent:true});
map.addLayers([wms1,wms2,wms3]);
map.zoomToExtent(new OpenLayers.Bounds(123360.47,-162161.43,767730.83,418254.41));
map.setCenter(new OpenLayers.LonLat(8.80899,1.83925), 7);
map.addControl( new OpenLayers.Control.LayerSwitcher() );

LAYER
NAME "base"
STATUS ON
PROCESSING "LABEL_NO_CLIP=True"
CONNECTIONTYPE POSTGIS
CONNECTION "host=localhost user=postgres dbname=Guinea_Ecuatorial password=tph"
DATA "geom from gnq_africa_central using unique gid srid=32632"
METADATA
    WMS_NAME "base"
    WMS_SRS "EPSG:32632 EPSG:4326"
END
TYPE POLYGON
LABELITEM "na2_descri"
CLASS
    STYLE
        COLOR 185 122 87
        OUTLINECOLOR 0 0 0
    END
    LABEL
        TYPE TRUETYPE
        FONT "arial"
        SIZE 15
        COLOR 0 0 255
        ANTIALIAS TRUE
    END
END

LAYER
NAME "provincias"
STATUS ON
PROCESSING "LABEL_NO_CLIP=True"
CONNECTIONTYPE POSTGIS
CONNECTION "host=localhost user=postgres dbname=Guinea_Ecuatorial password=tph"
DATA "geom from gnq_provincia using unique gid srid=32632"
METADATA
    WMS_NAME "provincias"
    WMS_SRS "EPSG:32632 EPSG:4326"
END
TYPE POLYGON
LABELITEM "provincia"
CLASS
    STYLE
        COLOR 85 185 107
        OUTLINECOLOR 0 0 0
    END
    LABEL
        TYPE TRUETYPE
        FONT "arial"
        SIZE 15
        COLOR 255 0 128
        ANTIALIAS TRUE
        POSITION CC
    END
END
LAYER
NAME "municipios"
STATUS ON
PROCESSING "LABEL_NO_CLIP=True"
CONNECTIONTYPE POSTGIS
CONNECTION "host=localhost user=postgres dbname=Guinea_Ecuatorial password=tph"
DATA "geom from gnq_municipios using unique gid srid=32632"
METADATA
    WMS_NAME "municipios"
    WMS_SRS "EPSG:32632 EPSG:4326"
END
TYPE POLYGON
LABELITEM "distrito"
CLASS
    STYLE
        COLOR 85 185 107
        OUTLINECOLOR 0 0 0
    END
    LABEL
        TYPE TRUETYPE
        FONT "arial"
        SIZE 10
        ANTIALIAS ON
        COLOR 0 0 0
        POSITION CC
    END
END

...{layers: 'municipios'},
 {isBaseLayer:false, visibility:false,transparent:true,opacity:0.5}
);