The hypothalamic hormones, arginine-vasotocin (VT) and isotocin (IT), play central roles in osmoregulation and in the regulation of social behaviors including aggressive behavior in many vertebrates including fish. Here, we examined whether these hormones are associated with aggressive behavior in the mudskipper (Periophthalmus modestus). The mudskipper is an amphibious fish, which lives in the brackish water of river mouths and displays unique aggressive behavior. Upon introduction to each other in an experimental tank with aquatic and terrestrial areas, a pair of males can be classified as aggressive dominant or submissive subordinate based on the frequency of their aggressive acts, which is significantly higher in dominant male. Additionally, the length of stay in terrestrial area of dominant was longer than that of the subordinate. The latter remained in aquatic area almost throughout the period of behavioral observation. The expression of brain VT mRNA was significantly higher in subordinate than in dominant, whereas neither IT mRNA expression nor plasma cortisol level differed between subordinate and dominant male. On the other hand, an intracerebroventricular injection of VT increased aggressive behaviors in mudskippers. In addition to known roles of VT in mediation of aggressive behavior, these results may shed light on the role of endogenous VT toward water migration in submissive mudskippers. The amphibious fish is a valuable experimental model to observe the relationship between effects of central VT on the osmoregulation and social behavioral regulation in vertebrates. © 2013.