That is a highly impressive statistic, as with almost 3.5 million visitors that makes Saint Louis the 3rd most popular zoo in the United States. I have a list of the 36 zoos that receive a minimum of one million annual visitors and only Disney's Animal Kingdom and Busch Gardens Tampa Bay (both theme parks) now surpass Saint Louis.
Approximate Annual Attendance: 36 Zoos
Disney’s Animal Kingdom – 9.5 million
Busch Gardens Tampa – 4.3 million
Saint Louis Zoo – 3.5 million (free zoo)
San Diego Zoo – 3.3 million
Lincoln Park Zoo – 3 million (free zoo)
Columbus Zoo – 2.4 million
National Zoo – 2.3 million (free zoo)
Brookfield Zoo – 2.3 million
Como Park Zoo – 2.2 million (free zoo)
Bronx Zoo – 2.1 million
Denver Zoo – 2 million
Houston Zoo – 1.8 million
Oregon Zoo – 1.6 million
Omaha’s Henry Doorly Zoo – 1.6 million
Los Angeles Zoo – 1.6 million
Phoenix Zoo – 1.4 million
Cincinnati Zoo – 1.4 million
San Diego Zoo Safari Park – 1.3 million
Minnesota Zoo – 1.3 million
Milwaukee Zoo – 1.3 million
Philadelphia Zoo – 1.3 million
Cleveland Zoo – 1.3 million
Detroit Zoo – 1.2 million
Rio Grande Zoo – 1.2 million
Woodland Park Zoo – 1.2 million
Indianapolis Zoo – 1.1 million
Central Park Zoo – 1.1 million (on only 6 acres!)
Lowry Park Zoo – 1.1 million
Memphis Zoo – 1.1 million
Fort Worth Zoo – 1 million
Pittsburgh Zoo – 1 million
San Antonio Zoo – 1 million
Riverbanks Zoo – 1 million
Utah’s Hogle Zoo – 1 million
Toledo Zoo – 1 million
Louisville Zoo – 1 million