Premating behaviour of the free-living marine nematode Chromadorita tenuis consists of movements of the male towards the adult virgin female stimulated by a pheromone released by the female. The vaginal gland cells are suggested as possible site of pheromone production. Sperm cells or an accon~panying substance probably inhibit pheromone release after insemination. At close range, the attracted male shows pre-copulatory behaviour consisting of reverse locomotion which 1s not sexspecific since it also occurs between males. Mating itself consists of the male coiling the posterior part of the body around the female. The single ventral seta anterior to the male cloaca is suggested to act a s receptor of mechanical stimuli (vibrations, touch) involved during, or preceding, mating. Insemination results in fertilization, egg shell formation, oviposition and loss of the capacity to attract males. Females mate only once during their life. Adult virgin females are continuously attractive to males; ovulation proceeds continuously; eggs are deposited in the proximal chambers of the uterus. There is no egg shell formation, nor are the unfertilized eggs deposited in the substrate; they are resorbed. At the age of 32 d, virgin individuals of both sexes have as high a fecundity/fertilization capacity as newly matured 16-d olds.