Is it possible you don't actually need a radio button here? They're generally used for choosing between two or more options, rather than a yes / no to a single option (for which checkboxes are designed). But if you do want to use a radio button here, I think the answer is javascript. This should see you right:

<form name="mainform">
<input type=radio name="foo" onclick="bar()">
<script language="javascript">
var i=0;
function
bar()
{
i=Math.abs(i-1);
document.mainform.foo.checked=i;
}
</script>
</form>
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§ George Sherston