Resumo. Abstract: Aim The objectives of this study were to evaluate which allometric measurements of Leptonema larvae are most suitable in order to develop mathematical equations to describe biomass relationships for the population of this taxon in a reference condition headwater stream. Methods We measured four body dimensions (body length, interocular distance, horizontal width of cephalic capsule and vertical width of the cephalic capsule) of 65 Leptonema larvae, which were collected in February 2022, in the Taboões spring, Serra do Rola Moça State Park, Minas Gerais, using a Surber sampler. For the determination of allometric measurements, each individual was photographed under a dissecting stereomicroscope (Leica M80) equipped with a digital camera. Each photographed specimen's length was measured using the Motic Image Plus 2.0 software. After measuring the linear body dimension and direct measurement of the biomass, we used these values to calculate the length-mass mathematical equations. To the equations use power models: DM = a Lb, where a/b are constants, DM is the dry mass, L is the linear body dimension. Results Among body dimensions of Leptonema larvae, body length showed the greatest range of variation, with values ranging from 4.03 to 25.28 mm, followed by head capsule vertical width (0.51 - 2.69 mm), head capsule horizontal width (0.55 - 2.22 mm) and interocular distance (0.24 - 1.88 mm). Our results show that body length provided the best-fitting equation for estimating biomass (R2 = 0.90). However, we observed a close fit between the other allometric measures, including high coefficients of determination, head capsule horizontal width (R2 = 0.85), interocular distance (R2 = 0.82), head capsule vertical width (R2 = 0.78). Conclusions These results will be useful in providing the best allometric measurement and equations to estimate the biomass of Leptonema larvae from the tropics.