The salinity, temperature and pH tolerance of Procephalothrix simulus Iwata, 1952, were experimentally studied. In hypo-media, the nemerteans could survive 96 h in 3.3‰ solution at 10 °C (median lethal salinity [LS 50] was not determined at this temperature), and 96 h LS 50 were 7.3‰ and 13.5‰ at 20 °C and 30 °C, respectively. In hyper-media, 96 h LS 50 values were 53.9‰, 47.1‰ and 41.4‰ at 10 °C, 20 °C and 30 °C, respectively. The trend of body weight changes in diluted media indicated that this nemertean is a volume regulator. During a 96-h exposure in media at 0 °C, worms were thanatoid but could recover if the temperature was gradually elevated to 20 °C. In thermal tolerance experiments, the nemertean survived 96 h in seawater of 30 °C, and worms suffered high mortalities when the temperature exceeded 32 °C. Present results suggest that the interaction of temperature and salinity on the lethal effects on P. simulus is significant ( P < 0.05). Elevated temperature (range 10–30 °C) decreased the worm's solute tolerance, and elevated salinity (range 18–38‰) decreased the worm's thermal tolerance. The survival pH level for this nemertean ranged from 5.00 to 9.20.