The densely populated monazite bearing high level natural radiation (HLNR) areas of Kerala coast are inhabited for generations and provide unique opportunities of inves-tigating the genetic and health effects of continuous low-level radiation directly on human populations. The average per capita dose received by the population in the HLNR areas is about four times the normal background radiation level. The  radioactivity measured in the 55-km-long and 0.5-km-wide belt (varying from less than 1.0 to over 35.0 mGy/year) is primarily due to thorium content (and its decay products) ranging from 8% to 10%. The biological systems in the region are thus exposed to alpha, beta and gamma radiation from both external as well as internal exposures through ingestion, inhalation, etc. [1–3]. The total population in the study area is about 4,000,000 with approximately one third residing in the HLNR areas and the rest in the adjacent normallevel natural radiation (NLNR) areas. The Department of Atomic Energy established a field laboratory and launched a comprehensive programme to determine the possible biological and health effects of HLNR on human populations [3–5] in collaboration with the State Government of Kerala. As a part of this programme, cytogenetic investigations are being carried out to establish the incidence of various types of chromosomal.