first zoo in the United States, opened in Philadelphia, Pennsylvania, in 1874 with an animal inventory of several hundred native and exotic specimens. It was begun and continues to be operated by the Zoological Society of Philadelphia, founded in 1859. In 1868, three years after the end of the American Civil War, a 42-acre (17-hectare) site was selected in Fairmount Park, an architect was sent to study the London Zoo, and the collection was begun. The Philadelphia Zoo developed the first zoo laboratory (1901) and the first children's zoo (1938) in the United States. It was the first zoo to formulate specific diets for its animals (1930s), and one, monkey cake, is still used today by many zoos.

Modern developments include an elaborate hummingbird exhibit (1970) and a reptile house (1972) with naturally planted displays. Also in the 1970s, three major outdoor exhibits were built: Wolf Wood, Bear Country, and the multispecies African Plains. The Philadelphia Zoo maintains excellent cat and waterfowl collections and is famous for the longevity records of its animals. It has about 1,800 specimens representing more than 400 species.