(frigate tuna), is a commercially fortunate to have a large resource of tuna, little information is available on the biology, genetics and stock structure of. Hence, the present study was conducted to study the biology, genetics and stock structure of in coastal October 2015 to September 2017 from day-boats operating in of Sri Lanka. For this period, the length-weight relationship and Fulton's condition factor (K) calculated for using standard length measured in centimetres and weight measured in grams were shown as = 0.1091 3.3385 and 1.93, respectively, indicating a relatively healthy growth pattern and a comparatively unpolluted habitat. Studies on the stomach contents revealed that are non-selective carnivors, shrimps and cephalopods available in the surrounding waters. The fecundity of female was shown to be 48,056 to 267,000 eggs. The calculated GSI values showed that the peak spawning period for male extends from May to August and for the females from May to July. The phylogenetic analysis of the mitochondrial D-loop region sequences of 75 selected samples representing all the geographical regions found in the coastal waters of the Western, North be considered as a single stock.