Background: In the best-seller book "Why zebras don't get ulcers" [1] Robert Sapolsky argues that animals do not suffer from stress-related diseases like humans, because for them stress is episodic, while humans in contrast suffer from chronic psychological stress. In particular, the idea that fish cannot experience psychological stress is still prevalent, partly due to the lack of a homologous brain area to the neocortex. However, emerging evidence suggests that teleosts can undergo psychological stress, defined as a subjective and perceptual experience of the stressor, and in recent years the underlying mechanisms started to be unveiled., Summary: The occurrence of cognitive appraisal in the assessment of stressors has been demonstrated in fish, indicating that the subjective evaluation of stimulus valence and salience, rather than absolute intrinsic characteristics of the stimulus itself, play a key role in the activation of the stress response. Moreover, individual biases (i.e. cognitive bias) in the cognitive appraisal of stimuli have also been described in fish, with some individuals consistently evaluating ambiguous stimuli as positive (aka optimists) whereas other individuals (aka pessimists) appraise them as negative. As a result, optimists and pessimists show consistent differences in stress reactivity and susceptibility/ resilience to disease. Finally, social context has also been shown to modulate the response to aversive stimuli with the behavior of conspecifics either buffering or enhancing the response (i.e. social buffering vs. social contagion)., Key Messages: Cognitive appraisal of stressors occurs in fish, implying that the stress response is modulated by a subjective and perceptual experience of the stressor. Moreover, inter-individual consistent cognitive biases in the appraisal of stressors is also present in fish making some individuals more susceptible to stress-related diseases. Therefore, psychological stress has a health toll in fish and psychologically stressed fish can potentially have ulcers., (S. Karger AG, Basel.)