To study the relationship between weather parameters and the population of Helicoverpa armigera, cotton (cv. LRA- 5166) was grown during January, 2001 at Metakara, 24pgs, W.B. on an area of 500 m2, by adopting full package of practices recommended for the crop in W.B. condition. Result revealed that the H. armigera population has significantly negative correlation with maximum temperature (Max-T, r =-0.78) and significantly positive correlation with afternoon relative humidity (Min-RH, r = 0.80). Result revealed that 30.5 to 32.5 OC Max-T coupled with 65 to 77 % Afternoon-RH caused heavy infestation of the insect in cotton crop.