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The aim of the present study was to clarify the role of GH transgenesis and its production in social interactions in juvenile common carp Cyprinus carpi. With food pellets provided sequentially, the serum GH levels and behavioral effects were measured in 14 pairs of size-matched 'all-fish' GH-transgenic and non-transgenic common carp. In six consecutive observations during 3 days, transgenic fish had a higher movement level as well as a higher social status, being 2.69 times as aggressive, two minutes before and after the 10-min feeding session compared to non-transgenic fish. Transgenic fish also were more than 1.74 times as likely to consume each pellet. During the 8-day experiment, transgenic fish had 4.09 times higher specific growth rate in body weight as well as 6.36 times higher serum GH level than the non-transgenic fish. These results show that GH transgenesis promotes over-expression of GH and alters behaviors in juvenile common carp, thereby increasing their ability to compete and gain food resources, presumably to meet a higher intrinsic growth rate, which gives direct evidence for the GH-induced elevation in feeding competitive ability of GH-transgenic common carp. Understanding these relationships would not only help evaluating potential ecological effects of the escaped/released transgenic fishes, but also help using potential aquaculture of this growth-enhanced strain.