Population characteristics and mass culture of a brackish water turbellarian Macrostomum orthostylum were studied in the laboratory. Adult M. orthostylum has a maximum length of 1.5 mm and can survive for a maximum period of 112 days under controlled environmental conditions (24 plus or minus 1 degrees C; 12 h light: 12 h dark). It can tolerate a wide range of salinities from 0 to 30 ppt and reproduction occurred in 0 to 22 ppt. The euryhaline hermaphrodite M. orthostylum was observed to reproduce only sexually and have high regenerating power. It has a lifetime fecundity of 46 hatchlings per female short generation time (7 days), and rapid rate of laboratory population growth. It laid two kind of eggs, (1) subitaneous (non-resting) eggs are produced throughout the lifespan parthenogenetically and it increases the population size under normal environmental conditions, (2) dormant (resting) eggs are a product of sexual reproduction and are produced during adverse environmental conditions to ensure survival of the population over unfavourable periods. This characteristic enables it to exploit the highly unpredictable environment that natural backwater system offers. Mass culture experiment showed that under optimal laboratory conditions 4200 animals/l can be harvested at culture duration of 15 days.