The Antarctic runnel can be highly variable in space and time; however, this environment has provided an interesting habitat for terrestrial algal colonization. The runnel of Reeve Hill, Casey Station (66°16.855′S, 110°31.045′E), Antarctica, which occurs intermittently only during the brief austral summer showed high colonization by microalgae dominated mainly by Cyanobacteria mats and diatoms. 9 freshwater microalgae species which includes Oscillatoria fracta, Oscillatoria sp., Navicula muticopsis, Pinnularia borealis, Nitzschia sp., Prasiola crispa, Chlorococcum sp., Syenechococcus aeruginosus and Syenechococcus maior were recorded during the study. The algal community in the runnel to be observed closely resembles that in the larger running water ecosystems in the continent. However, intrusion of seawater, direct exposure to sunlight and influence of sea current in the lower part changes the species community. The lower runnel showed total dominance consists mainly of the marine planktonic Bacillariophycea. Other species detected includes Flagilaria kerguelensis, Flagilaria linearis, Eucampia antarctica, Thalassiosira tumida, Pseudonitzschia lineola, Pseudonitzschia turgiduloides, Achnanthes brevipes and Odontella litigiosa. Lower algal species diversity observed in this study suggests that the number of species obtained is related to the area of environment from which the samples were derived. The local assemblages simply contain species that is favored by the environmental conditions.