Only during certain special events
I think there a couple of days a year when it's free, not sure when. Regular adult admission is $5 and kids under 2 are free all the time.
I am not sure it is ever FREE There maybe days ehen tickets are discounted or someone is having a special cobranded event. I would suggest calling and or checking their web page and sign up for their newsletter and follow their facebook page. It is a great center.
1 day a year Turtle Day
There is a small area to the left that has a few exhibits that are free but the main area wasn't free when we went.
Just looked on the 28th it is
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