Here's a list of AZA-accredited zoos that offer free admission year-round.
Cabrillo Marine Aquarium
San Pedro, CA
Offering 10% discount at Gift Shop.
Cape May County Park Zoo
Cape May Courthouse, NJ
Offering 10% discount at Gift Shop.
Cosley Zoo
Wheaton, IL
Offering 10% discount at Gift Shop.
The David Traylor Zoo of Emporia
Emporia, KS
Offering 10% discount at Gift Shop.
Henry Vilas Zoo Madison, WI Offering 20% discount in the gift shop.
Hutchinson Zoo
Hutchinson, KS
Offering 20% discount in the gift shop.
Lincoln Park Zoo
Chicago, IL
Offering 10% discount on retail, food, education programs and ticketed public events.
Smithsonian National Zoological Park
Washington DC Offering 10% discount at Gift Shop.
Saint Louis Zoo
St. Louis, MO
Offering 10% discount on retail, food, education programs and ticketed public events.
Como Zoo and Conservatory
Saint Paul, MN
Offering 10% discount on retail, food, education programs and ticketed public events.
Salisbury Zoological Park
Salisbury, MD
Offering 10% discount on retail, food, education programs and ticketed public events.