Ah, you see, that was a bit of important information: The table is part of the template of a repeater control.

You need to get the repeater's ItemDataBound event. It's in there that you can check to see, first, the type of the row you are looking at (It could be ListItemType.Header, .Item, .Footer), and then if that condition is met, do a .FindControl() on the EventArg parameter of the event. (e.FindControl("tableid"))