server controls created on .aspx won't create on .aspx.cs pages

Hi,
I am stuck to a stupid problem: when I create server control on .aspx page
such as: <div id="blabla" runat="server"></div>
then when I try access "blabla" from server side (.aspx.cs) page, it says"blabla does not exist"..


There was no problem so far till when I enabled ajax on my project..Can it be somehow related to ajax?
I also removed from the .aspx page reference to AjaxToolkit and ScriptManager but no avail..
hope someone can point me right direction..
Thanks
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