||During mating males transfer not only sperm, but also protein and other substances in their seminal fluid. The functions of some seminal fluid components have been identified and these include aiding in sperm motility and protecting sperm from the harsh environment of the female reproductive tract. However, the function of most seminal fluid components are unknown. In this dissertation, I identify a seminal fluid protease, TRY-5, in Caenorhabditis elegans and show that it functions in sperm activation, a necessary step in production of functional sperm. C. elegans is a nematode that consists of two sexes: males, which produce only sperm, and hermaphrodites, which produce both sperm and eggs. During activation, each sperm cell develops a pseudopod, which allows sperm to migrate and fertilize oocytes. Male sperm are stored as nonmotile, nonactivated sperm, and little is known about the extracellular signals required for their activation. SWM-1, a protein with two trypsin inhibitor-like domains and a secretion signal, was identified as an inhibitor of male sperm activation. Here, I show a trypsin-like serine protease, TRY-5, is an extracellular signal for sperm activation necessary for sperm activation in swm-1 mutant males. While swm-1 mutant males contain prematurely activated sperm, swm-1 try-5 males contain nonactivated sperm. Despite being a signal for activation, try-5 is not required for male fertility, suggesting the existence of additional sperm activation signals. This alternative iv signal likely comes from hermaphrodites, since a known hermaphrodite sperm activation pathway is required for male sperm activation in the absence of TRY-5. To test whether TRY-5 functions in seminal fluid, I examined the localization of TRY-5 protein. A TRY-5::GFP fusion expressed under a try-5 promoter localized to several tissues of the somatic gonad that are involved in the storage and transfer of sperm, including the vas deferens and valve region. I followed TRY-5::GFP localization during mating and found that TRY-5::GFP was transferred to hermaphrodites, making TRY-5 a likely seminal fluid component. Transfer occurred in a consistent pattern, suggesting this process is well regulated. Overall, these results suggest that TRY-5 is a protease seminal fluid signal of sperm activation.