Postback fired each time not only ComboBox1_SelectedIndexChanged

Hellop, Here is my problem: Postback is fired each time I select an item in the comboBox. I thought that only the event method ComboBox1_SelectedIndexChanged() would be fired in Ajax mode. Here is my code :

Choice1 Choice2 Choice3
</form> using System; using System.Collections.Generic; using System.Linq; using System.Web; using System.Web.UI; using System.Web.UI.WebControls; using AjaxControlToolkit; public partial class _Default : System.Web.UI.Page { protected void Page_Load(object sender, EventArgs e) { Label2.Text += "Page load fired, "; if (!IsPostBack) { Label1.Text = "Hello"; } } protected void ComboBox1_SelectedIndexChanged(object sender, EventArgs e) { Label1.Text = ComboBox1.SelectedValue; } }