We report that full-length and truncated transcripts of Fyn tyrosine protein kinase are expressed during testicular development. Truncated Fyn (tr-Fyn) transcripts encode a 24 kDa protein with a N-terminal (NT) domain, a complete Src homology (SH) 3 domain and an incomplete SH2 domain. The kinase domain is missing in tr-Fyn. In contrast, full-length Fyn transcripts encode a 59-55 kDa protein. Fractionated spermatids by centrifugal elutriation express tr-Fyn transcripts and protein, but not full-length Fyn transcripts and protein. Neither full-length Fyn nor tr-Fyn transcripts and encoded proteins are detected in elutriated pachytene spermatocytes. Sertoli cells express full-length and truncated Fyn throughout testicular development. In contrast, sperm contain full-length Fyn transcripts and protein but not the truncated form. tr-Fyn protein is visualized at the cytosolic side of Golgi membranes, derived proacrosomal vesicles, along the outer acrosome membrane and the inner acrosome membrane-acroplaxome complex anchoring the acrosome to the spermatid nuclear envelope. Fyn and phosphotyrosine immunoreactivity coexist in the tail of capacitated sperm. During fertilization, the Fyn-containing acroplaxome seen in the egg-bound and egg-fused sperm is no longer detected upon decondensation of the sperm nucleus. tr-Fyn expands the catalog of truncated tyrosine protein kinases expressed during spermiogenesis. We suggest that the NT and SH3 domains of tr-Fyn may recruit adaptor and effector proteins, in particular GTPase activating proteins, required for acrosome-acroplaxome biogenesis, acroplaxome F-actin dynamics and Sertoli cell function. During fertilization, full-length Fyn in the acroplaxome may contribute to a transient local signaling burst during the early events of sperm-egg interaction.