The process of colonization of Antarctic fellfield soils by micro- organisms has two phases: Firstly, the immigration, survival and establishment of microbial propagules thenlselves, and secondly the stabilization of the soil for subsequent colonization and establishment by mosses, lichens and invertebrates. Dominant amongst primary microbial colonizers are the phototrophic cyano- bacteria and algae. Not only do they introduce organic nutrients into the micro- habitat but they also have a structural function. Filaments of several dimensions frequently form a mesh over the surface of the soil. This mesh often has a canopy structure of fine filaments closely admixed with the mineral soil grains, overlain by a layer of broader, longer filaments. This structure may provide a rich grazing zone for micro-invertebrates such as protozoa and nematodes. Microbial filaments, clusters and unicells frequently have mucilaginous sheaths or capsules which may cement mineral grains together and improve soil crust stability. The combination of filaments and mucigel promotes the formation of microbial “rafts” which are dispersed by wind or water. The diverse microbiota of these compound propa- gules is likely to improve their chances of successful colonization of unpopulated soil surfaces. Phototrophic microbes can be distinguished in mixed natural com- munities of undisturbed soil crusts by selective filtration of their autofluorescence spectra. Heterotrophs can be distinguished after staining. The population can be selectively quantified by television image analysis (TVIA) using criteria of pig- mentation, size and morphology.