The startle response is a response to sudden, startling stimuli, such as sudden noise or threatening disturbance. Fast escape is a common type of stress response behavior in teleost and amphibian larvae. Pufferfish (Takifugu obscurus), an important aquaculture species in southern China, typically exhibit an energy-dissipative inflation behavior in response to a stressor. It is unknown whether pufferfish have a fast escape capacity and under what kind of stressful situation. Understanding the stress responses and neuro-mechanisms of avoidance behavior provides a theoretical basis for improving aquaculture practices. We exposed larval pufferfish and zebrafish (Danio rerio) to a sudden sound stimuli (100 Hz, 110 dB) and recorded their behavior using a high-speed (1 000 frames per second) digital camera. Both species attempted to escape when exposed to sudden auditory stimuli. However, the probability of exhibiting fast escape behavior was significantly lower in pufferfish than in zebrafish, and the latency of the behavior was significantly longer in the former than in the latter(P<0.01). However, the behavior probability was significantly increased and the latency reduced in pufferfish by treatment with a GABAA receptor inhibitor(P<0.05). The Mauthner cell in the hindbrain is believed to be the commander neuron of this behavior. Together with the reticulospinal neurons (e.g., Mid2cm and Mid3cm), Mauthner cells form the brainstem escape network that regulates the fast escape behavior. Our histological evaluation revealed that the morphological characters of the Mauthner cells in pufferfish differed from those in zebrafish and most other fish. The cells were oval-shape, and the dorsoventral/mediolateral axial ratio was significantly smaller than that in the zebrafish. In the pufferfish medulla, we were unable to define neurons that are readily visible in zebrafish, including the Mid2cm, Mid3cm, or the synaptic connections between the eighth nerve and Mauthner cell. Our results suggest that the Mauthner cell in pufferfish was less developed and the fast escape behavior may not be as important as in zebrafish. Instead, the pufferfish has developed a unique behavior characterized by sudden inflation and floating on the surface of the water as a defense mechanism under sudden stressful stimulation. Our results provide a basis for further physiological study of the neural mechanism underlying the stress response in pufferfish.