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An opportunity to test Coffman’s (1989) proposal that ecological heterogeneity is one of the main factors for chironomid species richness occurred when prolonged drought in southern Spain was broken by heavy rain. Chironomid assemblage composition was studied in two Mediterranean mountain first-order streams, one permanent and the other temporary. Samples were collected in the permanent stream over extreme hydrological conditions: 1991-95 with relatively low and stable flow, and 1996-97 with relatively high and unstable flow. The temporary stream flowed, and therefore was sampled, only during the second period. In the permanent stream, spate events resulted in a notable increase in species richness due to the arrival of 32 species and to the permanence of 81% of the species resident over the prespate period. This relatively high percentage suggests that chironomids, as a group, have important resilience properties. However, some species disappeared from the permanent stream after the spates and the considerable decrease in abundance of other species, shows that, at the species level, resilience may vary greatly within the Chironomidae. Most of the species that appeared in the temporary stream were the same as those of the permanent stream during the fast flow period despite the great ecological differences between the two streams under study, suggesting the key role of discharge in structuring chironomid assemblages. The outstanding exception was the fugitive species Orthocladius calvus, the most abundant species in the temporary stream during the initial successional stages but never collected in the permanent stream.