Fertilization is the process that leads to the formation of a diploid zygote from two haploid gametes. This is achieved through a complex series of cell-to-cell interactions between a sperm and an egg. The final event of fertilization is the fusion of the gametes' membranes, which allows the delivery of the sperm genetic material into the egg cytoplasm. In vivo studies in the laboratory mouse have led to the discovery of membrane proteins that are essential for the fusion process in both the sperm and egg. Specifically, the sperm protein Izumo1 was shown to be necessary for normal fertility. Izumo1-deficient spermatozoa fail to fuse with the egg plasma membrane. Izumo1 is a member of the Immunoglobulin Superfamily of proteins, which are known to be involved in cell adhesion. Here, we describe BART97b, a new mouse line with a recessive mutation that displays a fertilization block associated with a failure of sperm fusion. BART97b mutants carry a deletion that inactivates Spaca6, a previously uncharacterized gene expressed in testis. Similar to Izumo1, Spaca6 encodes an immunoglobulin-like protein. We propose that the Spaca6 gene product may, together with Izumo1, mediate sperm fusion by binding an as yet unidentified egg membrane receptor.