Unknown parameter name 'audit_trail' error on startup

I am running Oracle 9i.I set the parameter audit_trail=DB in the parameter file of the DB. I bounced the db for it to take effect...during startup following error occurs:
ORA-01078 - failure in processing system parameters
LRM-00101 - unknown parameter name 'audit_trail'

Any idea why this is coming??Any resolution??

Subhasish

Error says parameter is misspelled. Check again!

Try it with spaces around the equals = sign.