Six monitoring fields were selected in the northern part of West Java, Indonesia to identify the factors which influenced the seasonal fluctuations of population densities of soybean pod borers, Etiella zinckenella (Treitschke) and E. hobsoni (Butler). Data of patrol surveys in Central Java were used for comparative analyses on the data obtained from field census in West Java. In the northern part of West Java, the Etiella population density was high in the dry season, while in Central Java, in the wet season. The Etiella population density tended to become high in both areas, in case where large fields were planted with soybean and/or mung bean. The difference in the Etiella population densities between the two seasons could possibly be caused by the variations in the quantity of the host plant mass in the area in each season. The size of area which could be covered by one monitoring field to predict the Etiella occurrence was estimated at approximately 14,000 ha.