Background -Defining conserved molecular pathways in animal models of successful cardiac regeneration could yield insight into why adult mammals have inadequate cardiac regeneration after injury. Insight into the transcriptomic landscape of early cardiac regeneration from model organisms will shed light on evolutionarily conserved pathways in successful cardiac regeneration. Methods -Here we describe a cross-species transcriptomic screen in three model organisms for cardiac regeneration -axolotl, neonatal mice and zebrafish. Apical resection to remove ~10 - 20% of ventricular mass was carried out in these model organisms. RNA-seq analysis was performed on the hearts harvested at three time points - 12, 24 and 48 hours post-resection. Sham surgery was used as internal control. Results -Genes associated with inflammatory processes were found to be upregulated in a conserved manner. Complement receptors (activated by complement components, part of the innate immune system) were found to be highly upregulated in all three species. This approach revealed induction of gene expression for Complement 5a receptor1 (C5aR1) in the regenerating hearts of zebrafish, axolotls and mice. Inhibition of C5aR1 significantly attenuated the cardiomyocyte proliferative response to heart injury in all three species. Furthermore, following left ventricular apical resection, the cardiomyocyte proliferative response was abolished in mice with genetic deletion of C5aR1. Conclusions -These data reveal that activation of C5aR1 mediates an evolutionarily conserved response that promotes cardiomyocyte proliferation following cardiac injury and identify complement pathway activation as a common pathway of successful heart regeneration.