Hola
ZITAL
Te pongo un ejemplo:
Código PHP:
<html>
<head>
<script type="text/javascript">
function selecciona(frm) {
for (i=0; ele = frm.sel.options[i]; i++)
ele.selected = true;
}
</script>
</head>
<body>
<form>
<select name="sel" multiple="multiple">
<option>uno</option>
<option>dos</option>
<option>dos</option>
</select>
<input type="button" onclick="selecciona(this.form)" />
</body>
</html>
Saludos,
