Among the aquatic insects, the Plecoptera play an important role in decomposition and nutrient recycling. Moreover, they are important in the energy flow throughout the system, as they are food to other insects and vertebrates. The aim of this research was to study the biomass and diet dynamics of Anacroneuria during four months in the middle basin of the Gaira River. A total of 1284 organisms were collected and organized into five size classes (0.1−0.5, 0.6−1.0, 1.1−1.5, 1.6−2.0 y 2.1−2.5 cm). We found differences size classes (H’=100.3; p<0.01; n=644). The > 2 cm size class was not found during samplings 5 and 6. Abundance was significantly different among microhabitats; with the highest abundance observed in leaves (763 organisms) and the lowest in gravel stones (86 organisms). The cumulative biomass was 441.6 PSLC mg/m2, where the intermediate sizes (0.6−2,0 cm) were the largest contributors to the biomass of the genus (average AFDW: 215.53 ± 4.67 mg/m2), while the largest size (2.1–2.5 cm) had the lowest contribution (average AFDW: 48.90 ± 4.62 mg/m2). The was no significant correlation between gut contents and body size. The most common fragments in gut contents were animals and detritus. Biomass available to the Anacroneuria genus had provides a high energy content that was negatively affected by rainfall and discharges.