If you received the stock from your company, and the stock had any value at the time you received it, the company would have been required to add the value of the stock on the day you received it to your income.  That value would be your cost basis.  If the stock had no value when you received it , then your cost basis would be zero.  Unless the amount of the sale is insignificant (in which case you can just use $0.00) you should contact your former employer to find out the amount they added to your income for the stock.