Abstract: The length -weight relationship and condition factor (K) of Semiplotus semiplotus (McClelland, 1839), were studied for an economically important species of food fish from the Dikrong river system of mountainous state of Arunachal Pradesh, India. A total of 120 specimens were randomly collected using cast net and related contraptions from the water bodies and length (mm) and weight (g) of each of the specimen was measured following standard equipments. The result revealed that length- weight of the species followed the cube-law indicating an isometric growth pattern of the species in its natural habitat. The length –weight regression equation for the species in general can be expressed as Log W = -2.7259+3.04269logTLand the length-weight regression equations of males, females and juveniles respectively were Log W = -2.7242+ 3.0926log TL, Log W = -2.7635+ 3.2592 log TL and Log W = -1.2375 + 2.6135 log TL having r value respectively 0.90674 , 0.98107,0.959 for each cases . All these values indicate positive correlation between length and weight in Semiplotus semiplotus in regard to their sex as well as growth stages. General wellbeing of the fish is found to be good, as indicated by the values of condition factor, which were nearer to or greater than 1.