Molon writes: According to the original version of Reg T (1934), it's not legal to short a stock under $5.
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Reg T says its not legal to short a stock that is not marginable. The SEC determines which stocks are marginable although a broker may impose additional restrictions, for instance if a stock is thinly traded. Usually, if a stock is under $5 the SEC will classify it as non-marginable.
If a stock consistently trades above $5 and slips below $5 it may still be marginable if the SEC has not reclassified it as "not marginable". The broker may still say you can't short it. But it is legal to short it.
I have never shorted a stock that is under $5. Until recently I have not had reason to but now I am looking more into it.