FACTS: - You write a put. - It gets exercised on Dec 18th, 2011. - You are now long 1,000 shares @ $100 each ($100,000 of stock) and there is a $100 commission for the assignment. Your cost basis is $100,100.00. - You sell half of the shares on Dec 31, 2011 and the other half the next day on Jan 1st, 2012. QUESTION: When figuring out the cost basis for your 2011 taxes, how do you allocation the commission to the 500 shares that are taxable? Do you just assign all $100 to your 2011 basis ($50,100) since that is when it happened, and then none ($50,000) to next year's basis... or do you allocation a proportional amount ($50,500 for each year) in commission.