The phenomenon of sea thunderstorm effect on the nighttime records of VLF atmospherics at 10 and 20 kHz over the tropical station Calcutta (22¿34'N,88¿24'E) has been examined for the period June 1976 to May 1978. The disturbance reveals first an enhancement and then a large fall, and in between these two there is a nearly steady level. This variation is named the 'nighttime effect.' Important results obtained from the analysis of 204 such effects are that (1) most of the nighttime effects appear to cluster around midnight, (2) a high correlation exists between the duration of enhancement and fall, and (3) magnitudes of enhancement and fall are greater at 10 KHz than those at 20 kHz. All the characteristic features of this nighttime effect and their probable origins have been critically analyzed and discussed in relation to sea thunderstorms of the adjacent Bay of Bengal.