mysql2 error in production

As I need to upgrade a Rails app to 3.1 , I am trying to deploy it on
a remote production server ( Linux Debian, after installing Rails
3.0.9 under rvm with all needed gems including mysql2 version 0.2.18
used by the app)

after deployment, starting the app, I get a Load error w mysql2...

** [out :: www.mydomain.com] /var/www/vhosts/mydomain.com/rails/
testsbga/shared/bundle/ruby/1.9.1/gems/mysql2-0.2.18/lib/mysql2.rb:
9:in `require': libmysqlclient_r.so.15: cannot open shared object
file: No such file or directory - /var/www/vhosts/mydomain.com/rails/
testsbga/shared/bundle/ruby/1.9.1/gems/mysql2-0.2.18/lib/mysql2/
mysql2.so (LoadError)
** [out :: www.mydomain.com] from /var/www/vhosts/mydomain.com/rails/
testsbga/shared/bundle/ruby/1.9.1/gems/mysql2-0.2.18/lib/mysql2.rb:
9:in `<top (required)>'

I surely forgot something when install mysql2 w this gem set ...
but I don't see what ..

( I am using the same server w Rails 3.1 wo any problem...)

thanks for your feedback

Add to your Gemfile:
gem 'mysql'
then run `bundle`.

thanks , actually was an issue with rvm gem set, fixed ( I cloned my
localhost gem set ..)