||Two highly related Caenorhabditis species: the dioecious C. remanei and the androdioecious C. elegans are adopting two totally different mating systems. They are believed to have evolved from a common dioecious ancestral species. If so, what are the key events guiding the transition? To get a glimpse into this process, identification of characters present in dioecious species but absent in androdioecious species would be a key start. We have previously found that only the females of the dioecious species can produce sex pheromone to attract males. Yet, males from both dioecious and androdioecious species respond to this pheromone. In this study, two areas were focused: 1) What is the sex pheromone? 2) How do males perceive the sex pheromone? The sex pheromone is found to be a volatile and stable substance. By GC-MS analysis, a candidate chemical was detected as a specific peak in the GC spectra of samples with high male attraction activity. The second issue concerns the male perception pathway and is addressed by genetic and molecular approaches. Different developmental mutants of C. elegans with defects in cell lineages and cell fates are tested to determine if specific cellular components are required for pheromone perception in males. My findings showed that three different types of neurons are required for the pheromone perception. The progress of this genetic dissection exercise to uncover the sex pheromone perception pathway will be discussed in this thesis.