I can see why something like this would drive you crazy. Have you checked to see that the code page of each table is set correctly? I don't know if that could have an inpact but it is worth a try. Another thought- when this occurs if you quit foxpro and then restart it are the values according to the indexes the same?
A problem is a problem only as long as it has a possible solution. Lacking that, it becomes a FACT!
