FER-1 is required for fusion of specialized vesicles, called membranous organelles, with the sperm plasma membrane during Caenorhabditis elegans spermiogeneis. To investigate the role of FER-1 in membranous organelle fusion, I first examined ten fer-1 mutations and found that they all cause the same defect in membrane fusion. FER-1 and the ferlin protein family are membrane proteins with four to seven C2 domains which commonly mediate Ca2+-dependent lipid-processing events. Most of the fer-1 mutations fall within these C2 domains, showing that they have distinct, non-redundant functions. I found that membranous organelle fusion requires intracellular Ca2+ and that C2 domain mutations alter Ca2+ sensitivity. This suggests that the C2 domains are involved in Ca2+ sensing and further supports their independent function. Using two immunological approaches we found three FER-1 isoforms, two of which may arise from FER-1 by proteolysis. By both light and electron microscopy these FER-1 proteins are localized to membranous organelle membranes. Together, these results suggest that the ferlin family members may share a conserved mechanism to regulate cell-type specific membrane fusion.In Chapter III, I present additional results toward studying the function of FER-1 using several broad-based approaches. First, I present a bioinformatics analysis of FER-1 C2 domains and the preliminary results of their calcium-dependent phospholipid binding capabilities. Second, preliminary interactions found with individual FER-1 functional domains by a yeast-two hybrid screen are discussed. Lastly, I present results from a candidate-gene approach to identify additional regulators of MO fusion, the sperm-specific synaptobrevins.