What USA city opened up the first Zoo?

Oh, you're talking about the first one in the states---that would be---Philadelphia in 1874.

http://www.phillyzoo.org/

LOL um yes kelly. philadelphia is correct!

This question, or a similar one, comes up now and then in Trivia contests. Usually, it was phrased, "What is the first oo in the US". Sammy's question was phrased much better, more specific. The problems with just "the first oo" are shown below.

Lincoln Park Zoo in Chicago opened in 1868 or 69, but only with a pair of swans. It later added a bear about 1874. I don't know if that really qualifies as a zoo. There was another zoo chartered before Philadelphia's, but it didn't open before Philadelphia's. It might have been Cincinnati's. I'll try to remember to call them Monday.