Bradysia odoriphaga Yang and Zhang is the most serious pest of Chinese chive, Allium tuberosum Rottle ex Spreng. (Liliaceae) in North China. Pesticide residues in this vegetable are very high following excessive use of organophosphorate insecticides. Because there have been no reports on the sex pheromones of B. odoriphaga, sex pheromone and mating behavior of B. odoriphaga were investigated as a possibility of developing semiochemical-based monitoring and control of this pest. In laboratory bioassays, live B. odoriphaga virgin females stimulated 78% of males to vibrate their wings and 67% of males to attempt to mate. Methylene dichloride washes of female whole bodies and excised ovipositors also attracted males. In field test, many B. odoriphaga males were attracted to the traps containing live B. odoriphaga virgin females or methylene dichloride washes of female whole bodies. Most flies mated only once, while a few mated as many as six times. After mating, females were still attractive to males. Flies' sexual behavior showed a daily rhythm. The higher mating activity was from 2200 to 0600 h and the lower from 1200 to 1800 h. These results indicate the presence of a female sex pheromone in B. odoriphaga with the ovipostor as the most likely source of pheromone production. It is possible to collect the maximum amount of sex pheromone between 2200 and 0600 h. There is the possibility that this sex pheromone may be used to monitor and control of B. odoriphaga in the future.