Page.aspx
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
</div>
<p>
</p>
<asp:ListBox ID="ListBox1" runat="server">
<asp:ListItem Value="1">Text1</asp:ListItem>
<asp:ListItem Value="2">Text2</asp:ListItem>
<asp:ListItem Value="3">Text3</asp:ListItem>
</asp:ListBox>
</form>
</body>
</html>
Page.aspx.cs
ListBox1.SelectedIndex = ListBox1.Items.IndexOf(ListBox1.Items.FindByText("Text1"));