Abstract <p>An analysis of long-term data (2010–2023) on the toxicity of native bottom sediments (BSs) of the Gorky Reservoir obtained by biotesting, using larvae of <i>Chironomus riparius</i> Meigen, 1804, is given. The results indicate that the pollution of BSs and their uneven integral toxicity in the water area of the reservoir are in general preserved. The mean linear sizes of chironomid larvae are mainly significantly smaller than the control values. Significant differences were recorded for the mortality of chironomid larvae in 2016 and 2017 and for their linear sizes in 2017 and 2021. The mean toxicity indexes are in general lower than the control values, which indicates the toxicity of BSs. The toxicity indexes were maximal in 2011 and low in 2010 and 2017. The content of total Cr, Mn, Zn, As, Cd and Pb in BSs of the reservoir significantly exceeds their clarkes in the Earth’s crust, which indirectly indicates additional anthropogenic pollution. Concentrations of the elements increase downstream and reach maxima in the near-dam section of the reservoir (Chkalovsk station). The values of the geoaccumulation index and the pollution coefficient indicate an insignificant anthropogenic input of priority heavy metals (Cr, Ni, Cu, Zn, Cd and Pb) into the Gorky Reservoir. The highest values of the geoaccumulation index are recorded in BSs of the lake section of the reservoir. The pollution coefficient reflects the moderate contamination of BSs with Cd in the Kostroma extension and in the lake section of the reservoir. According to correlation analysis, the mortality of chironomid larvae is affected by P, Mn, Fe, Co, Zn, As and Bi, but a significant regression model is obtained only for Mn (<i>R</i>² = 0.513).</p>