SG,
In the table that the subform is based on, is there a 'MOS Desc' field?
There shouldn't be! The 'MOS Code' is all that goes in there, as the
"master list' of Codes and Descriptions will always be able to tell you
which Description is associated with the Code you have entered, exactly
like you are doing it. Make sense? Your expression would indicate that
'MOS Desc' is also the name of the table that is the 'master list' of
Codes and Descriptions, is this correct? Also, the textbox you are
entering this expression into its Control Source should not be named
'MOS Code' or 'MOS Desc', can you check this? And also, it seems to me
that you have the expression topsy-turvy. If I understand you
correctly, shouldn't it be like this?....
=DLookUp("[MOS Desc]","MOS Desc","[MOS Code]='" & [MOS Code] & "'")
.... or, if the Code is numerical, like this...
=DLookUp("[MOS Desc]","MOS Desc","[MOS Code]=" & [MOS Code])