It sounds as though one or more of the records have a Null value for
SchoolSystem. You should use the Nz function on that field as well:
Me.[County].Visible = (Nz(Me.[SchoolSystem], vbNullString) = "93030") Or _
(Nz(Me.[SchoolSystem], vbNullString) = "93040")
In fact, I'd recommend using Nz in all 6 lines.

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Doug Steele, Microsoft Access MVP
http://I.Am/DougSteele
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>I have a form that I have two fields I want to show or hide based on the
> value in another field. Previously I had one field that worked OK.
[quoted text clipped - 33 lines]
> Can anyone tell me what is wrong? I'm a novice at VBA.
> Thanks Don