<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{HTTP_HOST} ^(www.)?midominio.com$
RewriteCond %{REQUEST_URI} !^/midirectorio/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /midirectorio/$1
RewriteCond %{HTTP_HOST} ^(www.)?midominio.com$
RewriteRule ^(/)?$ midirectorio/index.php [L] 

RewriteBase /
RewriteRule ^midirectorio/index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /midirectorio/index.php [L]
</IfModule>