A method for operating an internal combustion engine with detection of the fuel used for injection is described. In the method, the elasticity modulus of the fuel to be injected is determined at a first and a second injection pressure. A difference value is calculated from the difference between the two elasticity modulus values related to the pressure difference and is compared with a differentiating value. The fuel being used is detected depending on whether the difference value is above or below the differentiation value. In particular, the method is used for differentiating diesel fuel EN590 and biodiesel.