Centrosome reduction during mouse spermiogenesis has been studied by immunofluorescent microscopy using anticentrin antibody (20H5) and TEM. Centrin is detected as two spots in round spermatids, corresponding to a pair of centrioles. In elongating spermatids, centrin spots colocalize with the centrioles in the neck region, while the perinuclear ring from which manchette microtubules arise, does not label with the antibody 20H5. The proximal centriole of the elongating spermatids develops a prominent adjunct, which assembles an aster of microtubules. TEM studies after immunogold labeling revealed that centrin is associated with the distal and the proximal centrioles, but not with the adjunct. Centrin labeling in the neck region diminishes after spermiation stage, although it is not completely lost from all testicular sperm. Mature epididymal sperm do not display centrin labeling. Mouse sperm lose both distal and proximal centrioles at maturity. Loss of centrin staining appears to correlate with the degeneration of centrioles during mouse spermiogenesis.