A method and a programmable logic controller (SPS) for verifying an application program in a failsafe programmable logic controller, wherein a signature (desired value) is generated using program modules or a complete application program when creating a program, and a copy of the signature is stored in the programmable logic controller and in an external component, respectively. Before the safety-oriented application program is started, the copy of the signature stored by the programmable logic controller is transmitted to the external component and is compared with the copy in the external component. In a further comparison, a signature (actual value) is generated using the content of the main memory of the programmable logic controller and using the actually loaded application program and is then compared with the local copy of the desired value of the signature. Starting of the actual application program is enabled only when both comparisons are positive.