In the vicinity of the Guadalquivir Marismas, Spain, M. genei is univoltine. The flight season extends from May to August, with a maximum of emergence in June. Hatching lasts about 40 min and takes place in the afternoon (85% between 12.00 and 16.00 hrs), at 20°C approximately. The sex-ratio at emergence during May-June determined on 1285 individuals was not statistically différent from 1, with a very small excess of 99, but the ratio varies significantly from month to month: 46.9% d in May, 53.3% dd in June. A possible explanation for the delay in emergence in the d population might be sought in a smaller compétitive capacity of the dd at the beginning of the hatching period under conditions of high larval density and food scarcity. Like emergence, reproductive activity begins at a late hour, about noon. Compared to the known Gomphidae, M. genei does not exhibit any ethological peculiarities. Its success in the Marismas zone seems closely related to the conservation of aquatic habitats with open sandy bottoms.