There are two likely possibilities: There is a fee charged to the account. Perhaps the bank imposes a monthly maintenance fee or minimum balance penalty, or there was a fee for some other service, such as getting checks printed. In any case, if the account's balance isn't sufficient to cover the fee, it would have overdrafted the account and triggered the overdraft fee. There may be an error. Perhaps you made a bookkeeping mistake, or the bank charged the account in error. You can determine this by checking your records against the bank's records transaction-by-transaction.