Oestrus Ovis is a common sheep parasite in the Mediterranean region. This study was carried out in the Ebro River Valley near Zaragoza (northeast Spain) using tracer animals to describe the seasons when infestation is more likely. Based on that information and an analysis of the evolution of the parasite within the host, we suggest the most appropriate time for treatment. Adult instars appeared in May until November and there was a diapause beginning in October-November and as least until February, so it is suggested than sheep be treated with larvicide in December.