Connect from a new psql to an old 6.0 postmaster

Moin,

I have two database servers. One running postgresql 6.0
and the new one running 6.5.1.

If I run "psql -h oldone -d oldone -u" from the newer
server and input username and password psql says:

Connection to database 'oldone' failed.
Failed to authenticate client as Postgres user 'olduser'
using authentication scheme 131072.

What does it mean and how can I fix this.

Many thanks
Matthias