Kevin,

>yes, I did. There IS a form open, ie _Screen.ActiveForm = .t.

There is a form open doesn't mean that _screen.ActiveForm is referencing any form at all or the form you think you are trying to manipulate. You've never said where the code you are trying to run is located. What happens when you do this:

? _screen.ActiveForm.Name
_screen.ActiveForm.TitleBar = 0

>I have made no other changes to properties which might effect
>this behavior.

Ummm, yes you have, because it's not working for you. Try this in the command window:

ox = createobject( "form" )
ox.Visible = .t.
ox.TitleBar = 0

does the titlebar disappear? (it does on my machine)

ox.TitleBar = 1

does it come back? (it does on my machine)

If so then there is some difference between your form and the VFP baseclass that is preventing the normal behavior from happening.