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system
what happens if stock purchased on margin plummets below what i have in the brokerage?
user
different brokerages have different house rules for margin requirements and margin calls.  you will likely get a margin call giving you a small amount of time to deposit the required funds to bring your account balance up to the required margin requirements.  in reality, a stock that falls from $50 to $4 in a short period will probably become unmarginable.  in short, yes, you will owe the broker for the loss.
assistant