We examined fluctuating asymmetry (FA) levels in fourth-instar larvae of Chironomus riparius(Diptera, Chironomidae) collected monthly from a polluted site in Galicia (NW Spain) where pollutant inputs are known to be roughly constant throughout the year. The site was selected because, despite this constancy in pollutant inputs, deformities in fourth-instar larvae were previously found to be more frequent and severe during cold periods and less frequent and severe during warm periods of the year, in accordance with the ‘time-of-exposure' hypothesis. This hypothesis predicts that short-term climatic variations occurring throughout the year influence the frequency and severity of larval morphological alterations, by means of the control they exert on developmental time and, as a consequence, on the time larvae remain exposed to pollutants. We investigated whether FA levels in larvae were likewise in accordance with this hypothesis, but, contrary to our expectations, no significant differences in FA levels were detected either among months or among seasons, suggesting that FA is not influenced by normal climatic variations. However, climatic influences may be masked, and the observed constancy in FA levels over the sampling period may be a consequence of the action of a mixture of stressors which compensate each other.