The Philadelphia Zoo is marking its 150th anniversary. The gates first opened on July 1, 1874.
More than 3,000 people came that day to see the first zoo of its kind in the country.
The official grand opening was delayed because of the Civil War.
It has since welcomed nearly 100 million guests, bringing them up close with wildlife and species from around the globe.
City leaders gathered at the zoon on Monday morning to mark the occasion.
Mayor Cherelle Parker issued a proclamation, declaring Monday to be "Philadelphia Zoo Day."
The public is invited to take part in the celebration, which includes music, food, performances, and crafts for the kids.
Action News reporter Katherine Scott has more on what the zoo has to offer.