Microdemographic studies are of great utility in correct understanding of fertility, mortality and migration, factors influencing population dynamics. Comparative studies of different ethnic groups residing in the same geographical region and under similar socioeconomic condition may be useful in spelling out the fertility and mortality differentials inherent in the groups. Present study was conducted in three different ethnic groups viz. Scheduled Caste, Scheduled Tribe and Muslims, residing in Baunia island in Sandeshkhali-1 Block in the 24 Pargaaas (South) District of West Bengal. The groups show close similarity in literacy, occupation and economic condition, but exhibit difference in fertility and mortality performances. Of the three groups, the Scheduled Tribe shows a higher reproductive wastage, probably with a loss of greater number of female fetuses, than the other groups. Apparently, no other extrinsic factor than greater dependance of the group on sundry labour accounts for such a high frequency of antenatal deaths, save of course any genetie difference.