The productivity or availability of juvenile habitats have theoretically been shown to determine size distributions of adult individuals and population abundance. Brown trout (Salmo trutta) is a species where adults may inhabit lakes and use connected streams for spawning and as nursery habitats for juveniles. This project aimed to estimate what factors determine the density and growth of juvenile brown trout in mountain streams. This was done by sampling 21 mountain streams for juvenile brown trout densities and growth, habitat availability and stream size, productivity, and resource availability. Additional data were obtained from other mountain streams from the Swedish Electrofishing Register. My results showed that measures of stream productivity and estimated benthic resource availability did not affect the densities and biomass of older juvenile brown trout. Instead, higher densities of older juvenile brown trout were related to increasing stream size (stream width x water depth). This is likely due to interference competition for space, where territory size influences the growth and densities of brown trout in streams but also that larger streams can provide refuges for larger individuals during periods of drying and thermal stress. This indicates that my estimate of stream size is an important factor determining densities of older juvenile brown trout in low-productive mountain streams. Stream size can be suggested to be a used as proxy for density and production of larger juvenile brown trout and therefore it may also be used to predict adult lake-living brown trout population size structure and abundance.