The sensitivity of the echonavigating system of Collocalia fuciphaga was tested under experimental conditions. In total darkness these swiftlets were able to fly and to avoid collision with the walls of the flight chamber, but apparently had difficulty in detecting a barrier of vertical wooden rods 1 cm square in crosssection, placed 15 cm apart. One Collocalia esculenta was also tested. It apparently lacked all ability to orientate in flight in darkness and was unable to avoid collision with the walls of the flight chamber.