The premiere zoo body in the North America is the Association of Zoos And Aquariums or AZA.
Association of Zoos and Aquariums
An alternative group formed several years ago is the Zoological Association of America or ZAA.
Welcome to ZAA.org
An even newer alternative group (which I just learned about) is the United States Zoological Association or USZA.
USZA | Official Site
As far as I can tell, the top tier zoos are in the AZA and the second tier are in the ZAA and the third tier (only about a dozen so far) are in the USZA. Snowleopard is correct, no question the AZA zoos are the best of the bunch. However, knowing just a little bit about the politics of the AZA (from what I hear as a volunteer at an AZA zoo and from a couple of acquaintances at non AZA zoos) I can fully understand why some zoo directors flatly refuse to join AZA.
Association of Zoos and Aquariums
An alternative group formed several years ago is the Zoological Association of America or ZAA.
Welcome to ZAA.org
An even newer alternative group (which I just learned about) is the United States Zoological Association or USZA.
USZA | Official Site
As far as I can tell, the top tier zoos are in the AZA and the second tier are in the ZAA and the third tier (only about a dozen so far) are in the USZA. Snowleopard is correct, no question the AZA zoos are the best of the bunch. However, knowing just a little bit about the politics of the AZA (from what I hear as a volunteer at an AZA zoo and from a couple of acquaintances at non AZA zoos) I can fully understand why some zoo directors flatly refuse to join AZA.