listing active directory Sub OU in an OU

So i am trying to list out the Sub-OU inside an OU but i am having problem doing it as it lists everything. I only want a sub OU inside a specified (hard coded) OU.

could someone please point me to the right direction? 

[code]

public void getOU()
    {

        string defaultNamingContext;
        //TODO 0 - Acquire and display the available OU's
        DirectoryEntry rootDSE = new DirectoryEntry("LDAP://RootDSE");
        defaultNamingContext = rootDSE.Properties["defaultNamingContext"].Value.ToString();
        DirectoryEntry entryToQuery = new DirectoryEntry("LDAP://" + defaultNamingContext);
        //MessageBox.Show(entryToQuery.Path.ToString());

        DirectorySearcher ouSearch = new DirectorySearcher(entryToQuery.Path);
        ouSearch.Filter = "(objectClass=organizationalUnit)";
        ouSearch.SearchScope = SearchScope.Subtree;
        ouSearch.PropertiesToLoad.Add("name");

        SearchResultCollection allOUS = ouSearch.FindAll();

        foreach (SearchResult oneResult in allOUS)
        {
            //comboBox1.Items.Add(oneResult.ToString());
            lstOU.Items.Add(oneResult.Properties["name"][0].ToString());
        }
} 

[/code]

i am calling the function from my page load event and this is the asp form.

<div>
                <asp:DropDownList ID="lstOU" runat="server">
                </asp:DropDownList>
            </div>

thanks