Ophthalmic surgical instrument, having: - an irrigation fluid line through which irrigation fluid can flow from an irrigation fluid container to an ophthalmic surgical handpiece for phacoemulsification, - a first fluid pump arranged, in a direction of flow, between the irrigation fluid container and the handpiece and configured to convey irrigation fluid to the handpiece, - a first volumetric flow determination device arranged, in the direction of flow, between the first fluid pump and the handpiece and configured to determine a first volumetric flow, - an aspiration fluid line through which aspiration fluid can flow from the handpiece to a collecting container, - a second fluid pump arranged, in the direction of flow, between the handpiece and the collecting container and configured to convey aspiration fluid to the collecting container, - a second volumetric flow determination device arranged, in the direction of flow, between the handpiece and the second fluid pump and configured to determine a second volumetric flow, characterized by - a difference element coupled to the first volumetric flow determination device and the second volumetric flow determination device and configured to form a difference from the first volumetric flow and the second volumetric flow, in order to form a differential volumetric flow signal, - a control device with a first input for receiving the differential volumetric flow signal and with a second input for receiving a signal for an irrigation fluid setpoint pressure, wherein the control device is configured to determine, from the differential volumetric flow signal and the irrigation fluid setpoint pressure signal, a signal for an irrigation fluid control pressure and to supply this signal at an output of the control device to the first fluid pump. - an irrigation fluid line through which irrigation fluid can flow from an irrigation fluid container to an ophthalmic surgical handpiece for phacoemulsification, - a first fluid pump arranged, in a direction of flow, between the irrigation fluid container and the handpiece and configured to convey irrigation fluid to the handpiece, - a first volumetric flow determination device arranged, in the direction of flow, between the first fluid pump and the handpiece and configured to determine a first volumetric flow, - an aspiration fluid line through which aspiration fluid can flow from the handpiece to a collecting container, - a second fluid pump arranged, in the direction of flow, between the handpiece and the collecting container and configured to convey aspiration fluid to the collecting container, - a second volumetric flow determination device arranged, in the direction of flow, between the handpiece and the second fluid pump and configured to determine a second volumetric flow, - a difference element coupled to the first volumetric flow determination device and the second volumetric flow determination device and configured to form a difference from the first volumetric flow and the second volumetric flow, in order to form a differential volumetric flow signal, - a control device with a first input for receiving the differential volumetric flow signal and with a second input for receiving a signal for an irrigation fluid setpoint pressure, wherein the control device is configured to determine, from the differential volumetric flow signal and the irrigation fluid setpoint pressure signal, a signal for an irrigation fluid control pressure and to supply this signal at an output of the control device to the first fluid pump.