Abstract. An operational data assimilation system for the Eastern Mediterranean is described and evaluated for a 6-month twin experiment. In the assimilative run, glider profiles of temperature and salinity are assimilated daily into a high resolution ocean forecast, after an initial spin up of one week. In the control run, the same initial and boundary conditions are used to produce an operational forecast, but without assimilation of in situ data. While both runs were similar for most of the time and most of the domain, significant differences were found near the region of assimilation, particularly when the glider passed through the anticyclonic Cyprus eddy. Root mean square differences of the misfits between the temperature and salinity observations and the model background field at those locations (before any assimilation) were approximately 15% lower in the assimilative run. Improvements in the forecasting capability of surface currents were found, and would provide a significant improvement of predictive capacity for applications such as pollutant spreading or offshore operational safety.