Genetic structure of four populations of Metapenaeus affinis from Maharashtra, Orissa, Kerala and Tamil Nadu in India was studied using RAPD markers. Five selective primers provided distinct and consistent RAPD profiles in all the four populations. The bands in the range 225-1,900 bp were scored for consistent results. The RAPD profiles generated by all the five primers revealed varying degrees of polymorphism, ranging from 25.00% (primer E-03) to 65.00% (primer E-06). Nei's (Nei M, Natl Acad Sci Proc USA 70:3321-3323, 1973) genetic diversity (h) among the four populations varied from 0.2565 ± 0.2146 (Orissa population) to 0.3576 ± 0.1897 (Maharashtra population).