Could you tell me which stop to get off (St Charles Ave streetcar) to reach the zoo and is there a shuttle bus that runs to the entrance of the zoo. If so, how frequently? many thanks once again.

whoops... forgot to mention this is New Orleans

You get off at the Audubon Park/Loyola University stop. As for a shuttle bus, I don't know.

You'll see the University on your left when it's time to get off.
On the website it only says the shuttle runs regularly! When we got off the streetcar we did't see the shuttle and had a leisurely 15 min walk through the park to the zoo, it wasn't until we walked back to the streetcar stop stop that we saw the shuttle.
If you are going to go to the Aquarium or on a paddleboat look out for combi tickets which were good value for the attractions.

Regularly is really the way to describe the frequency of the shuttle. I would say there is probably one every 15 mins or so. I think there are usually 2 buses that just circle the park from the St. Charles stop to the zoo entrance stop continually. The park is beautiful to walk through so you don't really need the shuttle if you are not too tired out from walking at the zoo. Also, you can't miss the park stop on the streetcar. Audobon Park is HUGE and St. Charles runs along side of it for quite a ways - 4-6 blocks maybe??? We took the ferry boat (combo tix) up to the zoo and the street car back as we had just missed a departing ferry and didn't want to wait for the next. By the way, the combo tix can be purchased to include the aquarium which is really great and you don't have to do it all on the same day as the zoo. Combo tix are purchased at the boxoffice which is on the riverfront side of the aquarium.

Many thanks for your replies! I have bought combi tickets which include the Natchez, Aquarium, Imax and a city tour but wanted to see the zoo via the streetcar. It looks like we will probably take the leisurely walk through the park as it is so beautiful and get the shuttle back