You have to use columnwidth attribute of your AdvancedDataGrid tag.
like this: <mx:AdvancedDataGrid columnWidth="40%">
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Thanks for the quick reply...
This doesn't quite solve what I need to do....I need to be able to manipulate the individual percentage width of each of the 8 columns in my ADG. And, they range in widths from 5% to 20%. Setting the column width at the ADG level will apply that one width to all columns won't it?