Due to compatibility issues, I believe that you will have to use the export utility of the 8.1.7. the 10g database. Then you will have a dump file in the format of 8.1.7 and not the 10g, to be imported into the 8.1.7 database. Be mindful that any new feature of 10g that is being used in the 10g database will not be excepted in the 8i version.

Mark

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