Clara, when do you want to make it uneditable?  

Is the textbox being filled thru code and you never want the user to be able
to edit it? If so, Ruralguy's suggestion will do the job.

Or do you want the user to be able to fill in data for the textbox in a new
record but not be able to edit the data at a later time? In this case
something like this:

Private Sub Form_Current()
 If Not IsNull(Me.txtYourBox) Then
    Me.txtYourBox.Locked = True
 Else: Me.txtYourBoxB.Locked = False
 End If
End Sub

where txtYourBox is the actuak name of your textbox.