Twenty marine and one freshwater strain, belonging to the genera Oscillatoria, Phormidium, Plectonema and Spirulina (sensu GEITLER), exhibiting a cell width less than 3-mu-m were cultivated in different light intensities, temperatures and salinities. The study of the morphological variability allows one to determine the stable and variable morphological characters. The tolerances to the culture conditions, the possibility of survival in unfavourable conditions and the capacity of chromatic adaptation were investigated. The autecological characters are discussed in relation to the original habitat as well as to the taxonomic grouping of the strains based on the morphology. ; Peer reviewed