1. The relationship between respiration (by telemetry), wing-beat (by cinematography) and ultrasonic pulse emission (by telephony) was studied in the echo-locating bat, Phyllostomus hastatus, at rest and when flying in a large enclosure.

2. In resting bats echo-locative pulses were produced at almost any point in the respiratory cycle, in flying bats more frequently near the beginning and/or end of expiration.

3. During flight the respiratory cycle showed a one-to-one relationship with the wing-beat cycle, both having a frequency of about 10 c/sec. Expiration was associated with the upward recovery stroke of the wings.

4. Both resting and flying bats emitted up to four pulses per respiratory cycle. Grouping of pulses was prominent during flight, the first pulse in each group being normally emitted at the start of expiration, with the last pulse and the silent period between groups being associated with inspiration.

5. Momentary inflexions in the respiratory signal associated with pulse production may represent momentary reversals in the direction of air flow, or ‘mini-breaths’, to permit vocalization. This and other evidence suggests that the production of pulses during the inspiratory phase is probably accompanied by a very brief expiration.

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