Anzasca Valley is part of the Monte Rosa gold district located in the Italian Central Alps. Since we do not know the effects of arsenic on the aquatic environment in Anzasca Valley, we investigated the biofilm of three streams. The three perennial streams studied are in the Anza catchment but with different lithology. Rio Rosso flows out of an ancient gold mine (Miniera dei Cani); its waters, acid mine drainages, are rich in iron and arsenic. Rio Gattera, a small stream adjacent to the Rio Rosso, flows through metamorphic rocks with mixed composition. Its waters are not acidic but contain a small amount of arsenic. Rio Roletto is upstream, with respect to the others, and drains different metamorphic rocks without arsenic. We analyzed the chemistry and the metals of the water, characterized by microscopic analysis and HPLC, the phytobenthic community living in the biofilm, and in the Rio Rosso, we measured the arsenic adsorbed in the biofilm. Sampling was performed between 2012 and 2014, and arsenic in the biofilm of Rio Rosso was measured in different seasons. In the three streams, the carotenoids of the biofilms showed the different stability of phytobenthic communities (Bacyllariophyceae vs. Cyanobacteria): in Rio Roletto and Rio Gattera, the ratio between the communities did not change; in Rio Rosso, the ratio between the communities changed completely, probably due to the peculiarity of water composition and presence of arsenic.