CatSper family proteins are putative ion channels expressed exclusively in membranes of the sperm flagellum and required for male fertility. Here, we show that mouse CatSper1 is essential for depolarization-evoked Ca²⁺ entry and for hyperactivated movement, a key flagellar function. CatSper1 is not needed for other developmental landmarks, including regional distributions of Ca_V1.2, Ca_V2.2, and Ca_V2.3 ion channel proteins, the cAMP-mediated activation of motility by \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{HCO}}_{3}^{-}\end{equation*}\end{document}, and the protein phosphorylation cascade of sperm capacitation. We propose that CatSper1 functions as a voltage-gated Ca²⁺ channel that controls Ca²⁺ entry to mediate the hyperactivated motility needed late in the preparation of sperm for fertilization.