Long-Evans rats (Rattus norvegicus; ns = 10 males and 10 females) in a burrow system responded to a cat in the open area by retreating to a burrow and emitting ultrasounds of 18-27 kHz. Females made more frequent ultrasonic cries, with longer durations of ultrasounds. In a 2nd study (ns = 19 males and 19 females), sonographic analyses confirmed the more frequent vocalizations of females and indicated that the sound pulses of females were reliably shorter in duration and of higher base frequency than those of males. Also, females emitted more pulses per pulse train with shorter within-train interpulse intervals. Six basic pulse forms were determined, and males emitted more (70%) pulses with negatively accelerated descending frequencies than females (25%). The findings indicate that female rats show qualitatively different antipredator vocalizations than do males and add to previous findings of higher levels of female antipredator defensiveness.