RuntimeError (Mysql not loaded)

Any idea why mysql gem would not load properly ?

log/production.log:

RuntimeError (Mysql not loaded):
    /usr/lib/ruby/gems/1.8/gems/activerecord-2.1.2/lib/active_record/
connection_adapters/mysql_adapter.rb:7:in `define_all_hashes_method!'

[XXXX]$ irb
irb(main):001:0> require 'rubygems'
=> true
irb(main):002:0> require 'mysql'
=> true

Thanks,
Prem.