Abstract: In accordance with the EuropaBio Harmonised IRM plan (September, 2012) the baseline susceptibility of Ostrinia nubilalis (ECB) to the Cry1Ab protein needs to be established after which subsequent routine monitoring for changes in susceptibility should be carried out. The objective is to detect, in a timely manner, shifts relative to baseline susceptibility that could result in inadequate protection against the target species. This program will enable early detection of potential development of resistance in O. nubilalis if it occurs, and this will allow the proposal and implementation of additional risk mitigation measures. During 2005-2014, 14 areas with 140 samples of ECB were analysed. Thus far, susceptibility to Cry1Ab have been assessed for one laboratory colony and ECB collected in maize fields in Czech Republic, France, Germany, Italy, Hungary, Slovakia, Poland, Portugal, Romania, and Spain. ECB larvae were exposed to artificial diet treated with increasing Cry1Ab concentrations, and mortality and growth inhibition were evaluated after 7 days. Variation in Cry1Ab susceptibility (MIC50) of field samples was up to 13.1-fold. A smaller variability was found for ECB pooled according to geographic and climatic conditions (up to 6.6-fold). It was planned that all O. nubilalis larvae from field collections that survived the bioassay at the highest dose should be transferred to plastic boxes in groups of approximately 50 larvae, provided with newly detached MON 810 maize leaves, and fed ad libitum to record any survivors. As for the seasons reported here no surviving larvae were found after 10 days and thus confirmatory experiments were not conducted.