<asp:DropDownList ID="ddlDept" runat="server" DataSourceID="SqlDataSource1"
DataTextField="departments" DataValueField="DeptID" Font-Underline="False">
</asp:DropDownList>
<asp:SqlDataSource ID="SqlDataSource1" runat="server"
ConnectionString="<%$ ConnectionStrings:EmployeeDetailsConnectionString4 %>"
SelectCommand="SELECT DISTINCT [DeptID],departments FROM [Department]">
</asp:SqlDataSource>
in code behind do something like it
ddlDept.Items.Insert(0, "--select a dept value");