Most marine mammal strandings coincident with naval sonar exercises have involved Cuvier’s beaked whales (Ziphius cavirostris). We recorded acoustic and animal movement data on two tagged Ziphius and obtained the first direct measurements of behavioural responses of this species to mid-frequency active (MFA) sonar signals. Each recording included a 30-minute playback (one 1.6-s simulated MFA sonar signal repeated every 25 s); one whale was also incidentally exposed to MFA sonar from distant naval exercises. Whales...