If you have tickets to Hershey Amusement Park the Zoo is included. There is a bridge that connects the Amusement Park to the Zoo. You do need to get your hand stamped so you can return to the Amusement Park. They will stamp your hand as you cross the bridge.
If you have Hershey park tickets it's free. Not sure otherwise
What's hours for weekend of Sept 13 to 15
The Zoo is included in the Hershey Amusement Park which you access to either by buying tickets or seasonal pass.
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