Research conducted in 2007–2008 on eight sites designated along the Wisłok River showed great diatoms diversity – indicated a total of 401 diatom taxa. Achnathidium pyrenaicum, A. minutissimum var. minutissimum, Navicula gregaria, N. lanceolata, Amphora pediculus, Cyclotella meneghiniana, Gomphonema olivaceum var. olivaceum, Nitzschia dissipata ssp. dissipata were most frequent. Based on diatomaceous indices IPS, GDI and TDI has been shown that the upper part of Wisłok had a high and good ecological status. However, the middle and lower section of river was characterized by moderate and poor ecological status, on the base of the IPS and GDI indices (III and IV class). TDI value indicated bad water quality (V class) in the middle and lower section.