The population of white fly (Bemesia tabaci Genn.) on tomato in terai region of West Bengal was studied between 1997 and 1998. The population reached the highest level (1.68/plant) during middle of February and high level was maintained. from mid February to mid March when temperature, relative humidity, sunshine hr/day and rainfall were ranged from 17.07–22.13°C, 65.29–72.78%, 7.79–8.98 hr/day and 5mm respectively. Temperature, relative humidity and rainfall was found negatively correlated with white fly population. Under laboratory conditions the pest was biologically more active during October-November. All the development stages was recorded longer in December-January and being shorter during October-November. Duration of life cycle was longer (40 days) during December-January when temperature and relative humidity was recorded 21.13–25.60°C and 60.58% respectively and being shorter during October-November (19.57 days) at 26.98–29.20°C, temperature and 71.87% relative humidity. Duration,of different life stages and' thereby life cycle was found negatively correlated with temperature and relative humidity indicating the pest was more biologically active during dryer part of the crop growing season.