Try This ...
<script type="text/javascript">
function isNumberic(evt) {
var charCode = (evt.which) ? evt.which : event.keyCode
if (charCode > 31 && (charCode < 48 || charCode > 57))
return false;
return true;
}
</script>
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<asp:textbox id="txtEmpNo" runat="server" onkeypress="return isNumberic(event)">
</asp:textbox>