In July 1993 we made the following observations which, due to their faunislic interest, are worthwhile to be brougth on record here. O. costae was seen on 28 July on the Rio Alcanadre at Onlinena (Provincia Huesca, Aragon). Along a 50 m stretch we estimated roughly 10-20 individuals including one tandem. A specimen was caught and photographed. The dragonflies most often sat on some riverside vegetation. head facing the water. Sometimes we saw them sitting also on the gravel. Other dragonflies seen at the site were Platycnemis latipes, Ischnura sp.. Sympetrum fonscolombii and Onychogomphus forcipatus (I 6 only). The Rio Alcanadre is about 20 m wide and had at the time of our visit very brown, though not stinky waters. Large parts of the bankside are covered with vegetation, only a short stretch is an open gravelly bank.