The stress response is a natural physiological reaction necessary for adapting to an ever-changing environment, but if extended for a long period it can lead to chronic stress and to negative consequences for the organism. Fish manifest different reactions to stress and different coping styles, leading to vulnerability or resilience to stress. Resilience is the ability to deal with chronic stress, limiting the negative symptoms that it can cause and keeping a physiological and behavioral functioning comparable to that of individuals that are not exposed to stress. The overall objective of the present study is to investigate stress resilience in larval stages of the zebrafish Danio rerio, which have been exposed to a chronic unpredictable stress protocol. Stress resilience was assessed by measuring basal and stressor-induced changes in the exploration of a novel environment (novel tank test assay). This thesis is divided in three aims, with correspondent experiments:1)The novel tank test, a behavioral assay broadly used to measure anxiety-like behavior in adult fish, has not previously been used with fish of 14 and 15 dpf. The first aim, therefore, consisted in validating the use of the novel tank test for measuring difference in anxiety levels in two-weeks-old fish from different lines, tested individually. According to previous studies, adult wild-derived zebrafish are more anxious than domesticated lines, therefore it was hypothesized that wild-derived zebrafish would display higher anxiety in the novel tank test compared to their domesticated conspecifics and nacre. As expected, wild-derived fish of generation four, the ones closer to the fish found in nature, showed higher anxiety, compared to the other fish tested. 2) The chronic unpredictable stress (CUS) modified for larvae, a paradigm that uses mild stressors administered for eight days in a random fashion, was utilized to stress nacre fish of six dpf, which after the end of the CUS were tested together with controls, at 14 and 15 dpf, to assess their anxiety in response to stress. In line with previous studies, it was expected that CUS-stressed fish would be more anxious than their control counterparts, and this is what was found. These findings show that the CUS is suitable to stress six-to 13-dpf fish. 3) CUS-exposed fish and their controls, after the end of the CUS protocol, were subjected to an acute stressor and then tested, 15 or 45 minutes from the end of the stressor, in the individual novel tank test at 14 and 15 dpf. Consistently with published studies, it was expected that chronically stressed fish, given their previous experience with stress, would have a lesser response to the acute stressor than the controls. Moreover, the CUS-stressed fish were expected to show interindividual variability in their stress response, with some individuals more resilient and some more vulnerable, and it was hypothesized that this difference would emerge more strongly when fish had more time to recover (45 minutes) from the acute stressor. No significant difference was found, thus in this study the two-weeks old chronically stressed fish do not show a behavior attributable to a resilient or vulnerable phenotype. Overall, these experiments show that the individual novel tank test is suitable to assess the anxiety-like behavior of two weeks old zebrafish. Moreover, the CUS is a valid protocol to stress fish from six dpf, but chronically stressed fish of 14 and 15 dpf exposed to acute stress do not show clear signs of resilience.