As the primary producer in streams, periphyton plays an important ecological role in nutrient cycling processes, and has been widely used as an indicator for stream assessment and management. This study examined altitudinal patterns of stream periphyton biomass and their relations to environmental variables in three tributaries of the Lancang-Mekong River. Total dissolved solids, conductivity, and water temperature related negatively with elevation in the three streams. Periphyton biomass showed distinct responses to environmental variables in three streams, and no environmental variables had significant relationship with periphyton biomass in all streams. Altitudinal patterns of periphyton biomass in three streams were uniform: it had a positive relation with elevation, showing a consistent trend with the anthropogenic impact. The altitudinal pattern of stream periphyton biomass could be an ideal indicator to assess stream ecological condition, and monitor anthropogenic impact on mountain streams in the Lancang-Mekong River system. (C) 2014 Elsevier Ltd and INQUA. All rights reserved.