Technical Abstract: Despite advances in medicine and crop genetics, nematodes remain significant human pathogens and agricultural pests. This warrants investigation of alternative strategies for pest control, such as interference with pheromone-mediated reproduction. Because only two nematode species have had their pheromones identified, we sought to expand the known inventory of nematode pheromones. Our activity-guided purification identified the female sex pheromone of Panagrellus redivivus as the ascaroside ascr#1, a diapausal pheromone in the distant relative Caenorhabditis elegans. We thus hypothesized that ascarosides might constitute a general class of nematode pheromones and screened specifically for ascarosides across a diverse range of nematode species. We report that many nematodes produce species-specific ascaroside blends and that different nematode species are attracted or repelled by ascarosides. Our findings show that ascarosides are broadly used by nematodes, similar to quorum sensing signals in bacteria. This knowledge may enable new strategies for the control of parasitic nematodes.