Er...no I didn't. It worked in the test project, but the real application has a regular panel placed inside a flow layout panel, and it doesn't throw an error.... ....10 minutes later.... panelFlowLeft.Controls.Item("panelRight").Controls.Add(myUC) That works. The confusion was the flow layout control - the panel could not be referenced as an item of the main form, because it was 3 levels deep. Pretty simple huh? Well - its all learning, and the idea to create a new project is not a new one, but perhaps I can make a rule: if I take more than 20 minutes on an apparently simple problem that should work, isolate, and recreate! Happy Coding!