Can't get value from ListBox!?

 Hi,
 I have a form which contains a dynamically generated ListBox.  The code is like this:
form.aspx:
<form runat=server>
<asp:Label ID="message" runat="server" />
Some other text input boxes...
<asp:ListBox ID="productCategoryId" runat="server" />
<asp:Button ID="submit" Text="Submit" CssClass="submit" runat="server" OnClick="submitButton_Click" />

form.aspx.cs:
using System;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class productsubcategory_add : System.Web.UI.Page
{< ...