function ajax_liste_deroulante(f) {
  if (f.elements["secteur"].value == 'tout') {
     document.getElementById('activite').style.display = 'none';
  }
  var l1 = f.elements["secteur"];
  var l2 = f.elements["activite"];
  var index = l1.selectedIndex;
  if(index < 1)
     l2.options.length = 0;
  else {
     var xhr_object = null;

     if(window.XMLHttpRequest) // Firefox
        xhr_object = new XMLHttpRequest();
     else if(window.ActiveXObject) // Internet Explorer
        xhr_object = new ActiveXObject("Microsoft.XMLHTTP");
     else { // XMLHttpRequest non supporté par le navigateur
        alert("Votre navigateur ne supporte pas les objets XMLHTTPRequest...");
        return;
     }

     xhr_object.open("POST", "includes/ajax_activites.php", true);

     xhr_object.onreadystatechange = function() {
        if(xhr_object.readyState == 4) {
            document.getElementById('activite').style.display = 'block';
            eval(xhr_object.responseText);
        }
     }

     xhr_object.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
     var data = "secteur="+escape(l1.options[index].value)+"&form="+f.name+"&select=activite";
     xhr_object.send(data);
  }
}