It's going to cost customers of at least one online brokerage more to short some favored bear stocks. Starting next month, Harrisdirect, a division of Chicago-based Harris Bank, is going to begin charging customers for any additional costs the brokerage incurs in trying to execute a short sale in so-called hard-to-borrow stocks. Harris Bank is owned by Bank of Montreal ( BMO). In the past, Harrisdirect, like many retail brokerages, didn't pass on these extra charges to their short-selling customers. Rather, the brokerage absorbed the added charges, viewing them as a necessary cost of doing business. But some say the extra trading fee -- anything from a few to a few hundred dollars -- is a sign of things to come in the brokerage industry. That's especially so in light of a new Securities and Exchange Commission regulation that's aimed at cracking down on abusive short-selling practices in hard-to-borrow stocks. A hard-to-borrow stock typically is a smallish stock such as Martha Stewart Living ( MSO), NovaStar Financial ( NFI) and Netflix ( NFLX), with relatively few shares outstanding, many of which have already been sold short. On Jan. 3, the rules for shorting these and other hard-to-borrow stocks got a lot tougher under a new SEC measure called Regulation SHO. Regulation SHO doesn't alter the basics of short-selling, which remains a respectable way for traders to make money by borrowing shares from a broker and betting a stock will decline in price. However, the rule does make it more difficult for traders to establish new short positions in stocks that already are the subject of a high number of bearish bets. The rule prohibits brokers from permitting traders to short a stock unless there are "reasonable grounds" for believing there are shares available to borrow. If those shares cannot be later found, the rule sets down a procedure for brokers to close out the short positions in relatively short order.