Try the links below for info. Some of the advice may not be the most humane so pick and choose. I myself urge you to use the most humane method.
Also you can go to Google and type in ""humane wildlife removal" and "wildlife rescue organizations"
If you live in an area where you have opossums there should be some wildlife organization or group in your state, or maybe an adjoining state and you can call for advice or help.
http://www.wildlife-removal.com/possumpr...
http://www.humanewildlifecontrolsolution...
I don't know where you live, but in some areas there is an "Opossum Society". Also there may be a rehab wildlife person that could give you advice, or a national audubon society branch. Have you tried online like at:
http://opossumsocietyus.org/
I realized that a possum has made a nest in my shed. How can i get rid of these animals without hurting them?
