The annual transmission potential of Wuchereria bancrofti was calculated from the number of infective larvae found in the number of Culex quinquefasciatus expected to bite an individual exposed 24 hr a day for a year in Calcutta and in a rural (Memari) area. In Calcutta, an average of 151 mosquitoes/night/person were collected; it was calculated that 55,028 mosquitoes could bite a person per year, including an average of 177 infective mosquitoes containing 319 infective larvae of W. bancrofti. At Memari, an average of 284 mosquitoes/night/person were collected; here with the resulting calculation was that 103,718 mosquitoes could bite a person per year, including an average of 137 infective mosquitoes containing 223 infective larvae. The annual transmission potential was higher in the urban area than that in the rural area. Annual transmission potential is useful knowledge in the epidemiological assessment of transmission dynamics for the formulation of control strategies.