trying to install ruby on debian

[<felkinator-debian@yahoo.com>]

I'm trying to install rails on a debian system.
Once I get ruby installed and try to run gems to
upgrade gems (according to the instructions at
http://wiki.rubyonrails.com/rails/show/RailsOnDebianUnstable
I get 

ss40:~/source/rubygems-0.8.6# gem install rubygems
/usr/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:18:in
`require__': No such file to load -- zlib (LoadError)
        from
/usr/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:18:in
`require'
 
etc. You've all seen it.

libzlib-ruby1.8 *is* installed.

Any suggestions?