Is your rabbit shaking his head a lot and scratching at the base of his ears? When you look into his ear canals, do you see something akin to coffee grounds? If so, your bunny probably has ear mites, and you will want to get him treated right away. They can be painful and annoying for him, and the mites can spread easily to other nearby rabbits. Indeed, the crusty exudate can get so bad that the rabbit can no longer hold his ears up!
If your rabbit has a very mild case, you can wipe his ears with a cotton ball soaked with mineral oil. However, if there’s lots of that yucky, crusty stuff, DON”T wipe it out! It’ll hurt like heck. Instead, have your vet prescribe a dose of Ivermectin, which can be given either orally or by injection. Just one dose will usually clear things up within a few days, and the bunny can shake out the residue when the mites are dead. Your vet might prescribe Revolution, instead, but be sure to use the correct dosage, because it comes in two strengths and too much can kill your rabbit. See the medication page for more information.
Be sure to keep any mite-infested animals away from other animals so that they don’t spread. In addition, you should disinfect the rabbit’s pen, litter box, towels, bowls, and toys to clear the area of any eggs or pests lying about.