This is a surprisingly difficult question to answer. You should probably seek the advice of a professional (CPA, enrolled agent, or tax attorney) rather than relying on a volunteer forum, unless the amounts are small and you don't mind the risk. The problem is that there are conflicting court cases, so it requires a legal opinion based on your particular facts, possibly your location (the supreme court hasn't ruled) and the facts of the cases or fairly complicated.
Here are just a couple of articles. They may or may not be correct in any particular point.
https://www.thetaxadviser.com/issues/2016/feb/stock-received-in-demutualization-has-no-basis.html
http://www.costbasis.com/stocks/demutualizationshares.html
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