. 1.We propose a non-lethal sampling method involving stable isotope analysis for estimating the trophic position of the endangered giant water bug Kirkaldyia (=Lethocerus) deyrolli (Heteroptera: Belostomatidae) in the wild. 2.Kirkaldyia deyrolli individuals were collected and their 15N and 13C values were measured. The 15N and 13C values of periphyton and particulate organic matter, the basal food sources in lentic ecosystems of rice fields, were also measured to estimate the trophic position of K.deyrolli. When individual isotopic signatures of the whole body were compared with those of their middle leg tarsus, we found strong correlations between them for both 15N and 13C. To estimate their trophic position without killing individuals, we constructed a regression model incorporating their middle leg tarsus's isotopic signatures and their body size as explanatory variables. This non-lethal method revealed that K.deyrolli showed great individual variation in its 15N which is a proxy of trophic position, ranging from 5.60 parts per thousand to 8.11 parts per thousand. 3.To evaluate the negative effects of our non-lethal method on the fitness of K.deyrolli, we examined how the removal of the middle leg tarsus affected reproductive performance under laboratory conditions. A comparison between the manipulated and unmanipulated individuals revealed that the removal treatment did not have any negative effects on female clutch size or egg hatchability for males. 4.In conclusion, stable isotope analysis of the middle leg tarsus of K.deyrolli is useful for estimating its trophic position without lethal or any negative fitness effects.