C. elegans is sexually dimorphic, producing hermaphrodite and male sexes that differ from one another in both their morphology and behavior. The hermaphrodite has been studied extensively and over the past three years we have developed a web-based guide to its anatomy (www.wormatlas.org) designed to assist researchers with interests ranging from gene characterization to computational modeling. In addition, The Handbook, Glossary and selected Slideable Worm electron micrographs (EMs) from the site are to be published as a laboratory handbook in the coming year (with Cold Spring Harbor Laboratory Press, NY). As part of our long-term objective to describe C. elegans developmental stages and sexes, we are now embarking on a description of the male anatomy. Males differ from hermaphrodites primarily in the reproductive tract and in the tail, which bears the male copulatory apparatus (1, 2). The male has a single-armed gonad containing a germ line that produces only sperm. The tract opens to the exterior at the male anus (cloaca) via a modified rectal epithelial chamber called the proctodeum. The tail copulatory apparatus is organized around this opening and consists of the copulatory spicules, several types of male-specific external sensory organs, interneurons, motor neurons and muscles. Most male-specific cells arise post-embryonically through sex-specific division of precursors common to both sexes. Thus, establishing the adult male form involves the generation and organization of a large number of male-specific cells and their integration into an existing framework of non-sex-specific tissues. Several studies have assigned roles for male-specific cells in such fascinating sex-specific behaviors as mate-searching (3) and copulation (4). However, a full understanding of these behaviors requires a detailed knowledge of the cellular substrates underlying their expression, both at the level of individual cells and the functional units they form through their interconnection. In contrast to the hermaphrodite, the male anatomy is only partially described and, in particular, the connectivity of many male neurons is still unknown. To resolve this problem, we are currently collaborating with the Emmons lab to describe the fine structure and connectivity of individual neurons and other cell types in the male. This project resumes the effort initiated by the MRC in the 1970s to reconstruct the male posterior nervous system from the N2Y EM series. Emerging connectivity data will be incorporated into a web-based guide to the male anatomy. The male anatomy will be presented in a format similar to that used for the hermaphrodite. Additional features we hope to incorporate in the future include a Slideable Male Worm, 3D models of cell shapes generated from EM serial reconstructions and quick-time movies of those important events in a male's life. We anticipate that the male web pages will provide a comparative basis for gene expression or developmental studies and represent a significant advancement on current descriptions of this sex. It is our hope that, through this multi-tiered web-based description and an accompanying handbook, the workings of at least this male sex will become less of a mystery. 1. Horvitz and Sulston (1977) Dev. Biol. 56, 110-156. 2. Sulston et al. (1980) Dev. Biol. 78, 542-576. 3. Lipton and Emmons (2003) J. Neurobiol 54: 93-110. 4. Sternberg and Emmons (1997) In C. elegans II (Eds. Riddle et al., CSHL Press, NY), pp. 295-334.