EBITEIN1 is a recently identified extracellular signal-regulated kinase 2 (ERK2)-binding protein that is abundant in round spermatids. Here, I further characterized EBITEIN1. EBITEIN1 bound to nonphosphorylated and phosphorylated forms of ERK1 and ERK2. Phosphorylation and dephosphorylation experiments indicated that EBITEIN1 is usually phosphorylated in vivo and that it is a substrate of ERK2. The ERK2-binding domain was required for phosphorylation of EBITEIN1. Based on these results, I propose that EBITEIN1 is a phosphoprotein and a downstream interactor of ERK2 that participates in the intracellular signal transduction pathway mediating the morphogenetic development of round spermatids into spermatozoa.