The invention relates to a disposable bioreactor comprising a reversible, externally attachable sensor arrangement for measuring a physical variable of a contained medium. A sensor adapter (28) for receiving an electronic sensor arrangement (34, 38; 42; 44a, 44b, 46, 48) interacting with the medium cross-flowing the peripheral lines (14, 16, 18), by means of an inner delimiting surface (32a, 32b; 42; 44a, 44b, 46, 48) of the sensor adapter, is integrated into at least one of the peripheral lines (14, 16, 18) of the bioreactor which is used to supply or discharge the medium. Said sensor adapter (28) is mounted on the peripheral line (14, 16, 18) as an insert extending the peripheral line (14, 16, 18) and which can be cross-flown by the contained medium.