Background: Zebrafish have the ability for heart regeneration. However, another teleost animal model, the medaka, had not yet been investigated for this capacity. Results: Compared with zebrafish, the medaka heart responded differently to an injury: An excessive fibrotic response occurred in the medaka heart, and existing cardiomyocytes or cardiac progenitor cells remained dormant, resulting in no numerical difference between the uncut and injured heart with respect to the number of EdU-incorporated cardiomyocytes. The results obtained from the analysis of the medaka raldh2-GFP transgenic line showed a lack of raldh2 expression in the endocardium. Regarding periostin expression, the localization of medaka periostin-b, a marker of fibrillogenesis, in the medaka heart remained at the wound site at 30 dpa; whereas zebrafish periostin-b was no longer localized at the wound but was detected in the epicardium at that time. Conclusions: Compared with zebrafish heart regeneration, the medaka heart phenotypes suggest the possibility that the medaka could hardly regenerate its heart tissue or that these phenotypes for heart regeneration showed a delay. Developmental Dynamics 243:1106–1115, 2014. © 2014 Wiley Periodicals, Inc.