The distribution of apple snail Pomacea canaliculata are island-wide but Pomacea scalaris is only found in southern Taiwan. In order to gain more information on the not well-known alien invasive species, comparative studies on the reproduction and thermal tolerance of P. canaliculata and P. scalaris were conducted. Sexual dimorphism in shell morphology has been found in both species, with wider operculum in males. Positive correlation between shell length and penis sheath length or penis length has been observed in male P. canaliculata and P. scalaris, based on the samples collected during the period of December 2004 and March 2006, The width of penis sheath in P. canaliculata was greater than in P. scalaris. Positive correlation between shell length and the height and width of vestigial penis in female P. canaliculata and P. scalaris was also found. The width of vestigial penis in P. scalaris was greater than in P. canaliculata. Gonadosomatic index (GSI) in male P. canaliculata and P. scalaris was 68 and 60% and 31 and 33% in females. Their reproductive cycle was annual without seasonal peaks. Significant difference in thermopreferrenda was found between P. canaliculata and P. scalaris. The 24, 48 and 72-h lethal thermal minima temperatures in P. canaliculata and P. scalaris were similar, i.e. 9.8¡V11.8¢J. And, the 24, 48 and 72-h lethal thermal maxima temperatures were 33.1¡V35.9¢J. Based on the results, it is known that P. canaliculata and P. scalaris reproduce year-round and temperature is not a major factor in shaping the species distribution pattern in Taiwan.