In most versions of the Pokémon game, it is beneficial to catch an excellent Pokémon like Ditto, as he can transform into any kind of Pokémon, with the exception of the legendary, asexual Pokémon, Nidoqueen and Nidorina. If you want to catch Ditto in Pokemon Emerald, then you've come to the right place!
Step 1. Defeat Elite 4 with some powerful Pokémon
Step 2. Go to the Fossil Maniac home in Fallarbor, near Meteor Falls
You can get there through the town of Rustboro if you wish.
Step 3. Go to Desert Underpass
There you will surely find the following Pokémon:
- Ditto 50% Level 38-45
- Whismur 34% Level 35-38
- Loudred 16% Level 38-44