The 401k will become the estate's, since there is no specific beneficiary. Assuming that you are not just the only child but also that your father was not married when he passed away, you will inherit everything in the estate through "instate succession," or the rules for who gets what when  there is no will. Therefore, what you need to do is get the authority to act on the estate--to provide the necessary documentation (e.g. death certificate) to the company managing the 401k; to open an estate bank account; to deposit the 401 money into that account; then to distribute whatever is in the estate (the 401, plus anything else your father had) to you, as the sole heir. To get that authority, you need to be appointed the estate's "personal representative" by the court. Contact the probate. court's clerk's office to inquire into how to do this.