The co-existence between GM and non-GM crops is still a major goal of the EU policy. However, in some member states, like Spain, national co-existence strategy is still lacking and regional attempts to regulate this issue have been unfruitful. A previous field study in Mediterranean insular conditions suggested that an isolation distance of 30 m between GM and non-GM maize fields might be sufficient to keep the adventitious presence below the EU threshold of 0.9% (see Vives-Vallés contribution in this congress). Nevertheless, this isolation distance was obtained in an experimental design where cross-fertilization was favored in terms of the synchrony of flowering between GM and non-GM fields. In view of this, we are currently undertaking a field trial to study the effect of flowering de-synchrony on cross-fertilization between MON810 GM and non-GM maize in Mallorca. In particular, our aim is to determine whether the delay in flowering might be an effective strategy to improve coexistence. We expect that the results of this study will be useful for a future regulation on co-existence based on the best scientific data.