Hi
try the following
<asp:Repeater ID="Repeater1" runat="server" DataSourceID="SqlDataSource1" >
<ItemTemplate>
<asp:GridView ID="GridView1" runat="server" DataSourceID="SqlDataSource1" >
</asp:GridView>
<asp:TextBox id="txt_username" runat="server" > </asp:TextBox>
<asp:AutoCompleteExtender ID="AutoCompleteExtender1" runat="server" TargetControlID="txt_username" ServiceMethod="your method">
</asp:AutoCompleteExtender>
</ItemTemplate>
</asp:Repeater>
Hope this helps...