Microinjection in mouse eggs of tr-kit, a truncated form of the c-kit tyrosine kinase present in mouse spermatozoa, causes resumption of meiosis through activation of phospholipase Cgamma1 (PLCgamma1) and Ca(2+) mobilization from intracellular stores. We show that the Src-like kinase Fyn phosphorylates Tyr161 in tr-kit and that this residue is essential for tr-kit function. Fyn is localized in the cortex region underneath the plasma membrane in mouse oocytes. Using several approaches, we demonstrate that Fyn associates with tr-kit and that the interaction requires Tyr161. The interaction between tr-kit and Fyn triggers activation of the kinase as monitored by both autophosphorylation and phosphorylation of PLCgamma1. Co-injection of tr-kit with the SH2 domain of Fyn, or pre-treatment with a Fyn inhibitor, impairs oocyte activation, suggesting that activation of Fyn by tr-kit also occurs in vivo. Finally, microinjection of constitutively active Fyn triggers oocyte activation downstream of tr-kit but still requires PLC activity. We suggest that the mechanism by which tr-kit triggers resumption of meiosis of mouse eggs requires a functional interaction with Fyn and phosphorylation of PLCgamma1.