We observed the sound production and properties of fox jacopever (Sebastes vulpes) in a water tank experiment to obtain basic information for the Passive Acoustic Monitoring (PAM). Six pairs of experiments including mature and immature fishes were conducted, and sound properties with and without agonistic behavior were compared. Fox jacopever produced low frequency pulses under 2,000 Hz. Pulse number ranged between 1–77, pulse duration was 0.005–0.220 s, pulse period was 0.001–0.989 s, and peak frequency was 258–490 Hz. These properties were changed by the presence or absence of agonistic behavior. Higher peak frequency over 400 Hz was observed only when a pair included an immature fish. Our results showed the potential use of fish sounds for estimating size and status of territorial behavior in this species by PAM.