Trichoptera larvae are exclusively aquatic and they are the most abundant group of animals in the aquatic ecosystem of Plitvice lakes, where they primarily live on tufa barriers. Family Hydropsychidae are a part of the filtering fauna, a functional group that is extremely important in the overflow lake system, where overflowing water contains great amount of organic particles. Filtering fauna eliminate these particles from the water column and transform them into biomass available in the benthic community and in that way, they participate in transferring energy. In this study, a growth pattern and the amount of the secondary production of the hydropsychid larvae at two tufa barriers (lakes Milanovac and Novakovića brod) is presented. I have determined that the dominance of two species (Hydropsyche saxonica (McLachlan, 1884) and H. instabilis (Curtis, 1834)). When distinguishing five larval stages, I primarily used the width of the head capsule, but I also noted the length of the head capsule and body, and the mass, which was an important parameter in the calculation of secondary production. Most of the individuals were found in the moss layer. In the deeper tufa layer, young larvae have been found, which are primarily collectors, transferring to dominant mode of filter feeding later in life cycle. The most larvae of the first and second stages were recorded in July, thus marking the beginning of their cycle. H. saxonica was more abundant at Milanovac barrier, and H. instabilis on Novakovića brod barrier. Overall, at both locations H. saxonica dominated in abundance, size and consequently in the amount of secondary production. At the Milanovac barrier, H. saxonica exhibited grater abundances and much higher value of secondary production versus H. instabilis, probably due to its body size.