Custom Contol Can't Be Viewed

[quiggle]

I have created a custom control, a subclass of
System.Web.UI.WebControls.DataGrid which contains columns specific for
personnel data. The control works fine, but instances of it cannot be
viewed properly in Visual Studio in design view. Most of the properties
cannot be set either. Every instance says Error Creating Control and in the
tool tip it says "Parser Error:Unknown server tag.."

Any help would be much appreciated!!!

Aimee