Chironomid assemblages and sedimentological parameters (grain size, organic content, mineralogy) of a short sediment core from Lake Galletué (38°41' S, 71°17' W) were analysed. The sedimentary record includes one volcanic ash (tephra) layer, which has a completely different composition than the host sediment in terms of organic content, grain size, and mineralogy. According to the geochronology (210Pb and 137Cs), this ash layer corresponds to the eruption of Llaima Volcano in 1956-1957. The tephra deposition had an impact on chironomid assemblages producing, among other changes, an increase in Parakiefferiella and a decrease in Ablabesmyia, although no noticeable change was detected in the diversity index. When compared with other studies, our results also show an impact in terms of the presence of chironomid head capsules within the tephra. The presence of these head capsules could result from the effects of percolation, since the coarse grain size of the tephra particles provides large interstitial spaces. The recovery in the abundances of some taxa after the tephra input, suggests the lake is probably restoring the conditions prevailing before the tephra fall. © 2007 Sociedad de Biología de Chile.