The .dat extension is very common.. are you talking about MapPoint?

Try NotePad or WordPad first (you might also try a shareware program called TextPad).

Also, try importing it into Excel or Access specifying the file type as D-Base or dbf.

If you're still having no luck, try opening it with a binary editor and see if the file type is mentioned in the first several characters of the file..

best,
Eric

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