Problem with "NOT IN (subquery)

Hi,

I have a table named "famille" whose structure and content is :
famille_code | famille_mere_famille_code |  famille_libelle  |
famille_niveau
--------------+---------------------------+-------------------+-------------
---000000       |                           | Mhre              |
0I00001       | 000000                    | Composants        |
1IN0001       | I00001                    | Micro-processeurs |
2IN0002       | I00001                    | Mimoires RAM      |
2INS001       | IN0002                    | DDR-SDRAM         |
3INS002       | IN0002                    | DDR2-SDRAM        |
3INS003       | IN0002                    | SDR-SDRAM         |
3IN0003       | I00001                    | Cartes mhres      |
2IN0004       | I00001                    | Disques durs      |
2IN0005       | I00001                    | Cartes graphiques |
2IN0006       | I00001                    | Cartes son        |
2IN0007       | I00001                    | Riseau            |
2IN0008       | I00001                    | Lecteurs CD/DVD   |
2IN0009       | I00001                    | Graveurs CD/DVD   |
2IN0010       | I00001                    | Bontiers          |
2I00002       | 000000                    | Piriphiriques     |
1IN0011       | I00002                    | Cli USB           |
2IN0012       | I00002                    | Modems            |
2IN0013       | I00002                    | Imprimantes       |
2
(19 rows)

The first col is the family id and the second is the mother family id.
I would get a list of all families that are never in the col n°2, so
the families that aren't node but leaf.

The query, i made was "SELECT * FROM famille WHERE famille_code NOT IN
(SELECT DISTINCT famille_mere_famille_code FROM famille);"

But the DB returns 0 records instead of 15. If i use a list instead of a
subquery it works normaly but it's not easy to manage it like this.

So if anyone can help me please ...