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The distribution of Trichoptera of the Hozgarganta River (Los Alcornocales Natural Park, SW Spain) in relation with environmental factors was examined. Three groups of species were recognised according to the altitudinal gradient. In the headwaters the caddisflies Rhyacophila fonticola, Lepidostoma hirtum, Silonella aurata, Allogamus gibraltaricus, Hydropsyche infernalis and Diplectrona felix predominated; in the constrained section of the tributaries Polycentropus kingi, Chimarra marginata, Hydropsyche iberomaroccana, R. fonticola and Tinodes sp. prevailed; finally, in the main channel H. iberomaroccana, C. marginata, Hydropsyche lobata, Leptocerus lusitanicus and Rhyacophila munda were the most important species. A direct ordination analysis (CCA) was used to describe assemblage changes among sites and corroborated that conductivity and temperature were the variables that best explained Trichoptera distribution. The temporal analysis showed changes in the Trichoptera diversity and richness in permanent stretches, as well as variations in the structure of the communities according to the season. We identified autumn‐winter species (H. infernalis, H. siltalai, H. lobata, R. fonticola and R. munda ) and summer ones (Ithytrichia sp, Oxyethira unidentata, Mystacides azurea and Setodes argentipunctellus ). In the basin we distinguished permanent, intermittent and ephemeral reaches with similar caddisfly richness and diversity, however the species composition associated with each one was different. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)