Certain species of bats love to live in the attics or roofs of buildings. The most common species to do so are the Little Brown Bat (Myotis lucifugus) and the Big Brown Bat (Eptesicus fuscus) in the more northern parts of the US, and the Mexican Free-Tail Bat (Tadarida brasiliensis) in the southern areas. These bats are known as colonizing bats, because they tend to live in large groups, or colonies. The colony is often called a maternity colony, because it's almost entirely made up of female bats, for the purpose of birthing and raising young bats. These bats seek out a warm and safe place in which to give birth to and raise the young, and they often choose the attics of buildings to live in. Attics meet all of the criteria necessary for bats: they are high off the ground, often have only small entry holes (bats prefer such areas, because they're more safe from predators), and they're warm and dry. Colonizing bats seem to prefer very hot roosting areas. Thus, they often select attics. How then, does one get them out? You can't go into the attic and just grab them and get them out that way. They're all stuck in little crevices, and will flee as soon as you approach, and even if you did manage to get one, then what? You also can't trap them. See this page for more information about the ineffectiveness of bat trapping. In short, bat trapping is harmful to these beneficial creatures, very difficult to attempt, and pointless to actually solving the problem. Read the following page to learn why there are no effective bat repellents. Also, some people try to use high-pitch ultrasonic sound making machines, but read this page to learn why the FTC has declared them fraudulent and why high pitch sound machines don't work. There is certainly no such thing as an effective or legal bat poison. It's usually illegal to kill bats, and it'd be a disaster to try it. You wouldn't kill them all, and the ones that died would rot and create an odor problem and biohazard in your building. Luckily, it is possible to 100% completely and 100% effectively and legally remove a colony of bats living in an attic. The proper way to do it is via bat exclusion. You can't kill the bats, you can't relocate the bats (they'll migrate back no matter how far you take them), but you can get them all out and permanently seal them out. The basic principle is that they're excluded from the attic via one-way exit devices. Each job is different, each architecture is different, each colony is different, and time of year and other conditions can make the type of approach vary quite a bit. It's highly specialized work, and only a licensed professional with specific bat control training and years of experience should attempt bat control. When the bats are fully removed, it's vital that the building is 100% sealed against re-entry. Bats are creatures of habit, and after they're blocked out, they'll work very hard to get back in. They can enter extremely small gaps. If you leave just one area of a half-inch open, they'll find it and get back in - believe me, I know. It's absolutely amazing how small a hole they can squeeze through. Furthermore, it's often very important to clean up after the bats. If you've had bats living in your attic, they'll leave behind their droppings and urine. This is a problem not only because it creates an odor problem, but because the droppings can grow fungus that can cause the lung disease histoplasmosis in humans. It's often a good idea to have your attic cleaned if it's been inhabited by a colony of bats. http://www.247wildlife.com/bats-attic-rid.htm