Hi Ulises the FOR statment is used to evaluate in the destination table and not in the source table. If you want to do this way try the following
SELECT * FROM table_b WHERE FIELDX INTO CURSOR mycursor
SELECT table_a
APPEND FROM DBF('mycursor')

>A command as simple as:
>
>Select TABLE_A
>Append from TABLE_B FOR FIELDX=.T.
>
>it appends all records, not only FIELDX=.T.
>FIELDX in TABLE_B is a logical field.
>
>I tried also
>
>Append from TABLE_B FOR TABLE_B.FIELDX=.T.
>
>Same result.
>
>Why things apparently so simple are sometimes so complicated?
>
>TIA for your time.
>
>Ulises