Probá:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento sin título</title>
</head>
<body>
<form id="form1" name="form1" method="post" action="">
<select id="frut" name="select" onchange="alert(document.getElementById('frut')[selectedIndex].innerHTML)">
<option value="1">uno</option>
<option value="2">dos</option>
<option value="3">tres</option>
</select>
</form>
</body>
</html>