We describe design of echolocation signals in 5 species of aerial-hawking insectivorous bats from Malaysia. These bats forage in open spaces above the forest or in large clearings and belong to 2 families: Molossidae (Chaerephon johorensis, Mops mops, Cheiromeles torquatus) and Vespertilionidae (Hesperoptenus blanfordi, Pipistrellus stenopterus). As is typical for aerial-hawking bats, all 5 species produced narrowband calls of long duration (6–21 ms) and low peak frequency (16–44 kHz). However, sequences recorded from bats flying at high altitude (>10 m) were characterized by an alternation between calls that differed in frequency of maximum energy (peak frequency), switching between high- and low-frequency calls. In some species, the types of calls also differed in duration and sound pressure level. We consider possible implications of the alternation of types of calls for the detection of prey in open spaces.