Summary Numerous induced and spontaneous mutations may be detected on the algal cell colonies growing from the solitary cells on the surface of an agar medium. Lethal and sublethal mutations are represented by the dead cells. Cell minicolonies mostly show growth and developmental defects caused by some mutational damages of total genetic information. The colonies of large cells may consist of diploid, polyploid or aneuploid cells as well as of cells having a disturbed relation between the processes of growth and development. The pigment mutations are well perceptible by sight. Some mutations in the characteristics of the cell-wall or whole chlamys are recognizable by amoeboid spreading of the cell protoplast on the surface of agar. All the above mentioned mutations are well visible on the whole cell colonies, their sectors or in mosaic patterns. If the mutation expression is delayed, then the sectors are postponed from the centre to the margin of the cell colony. The groups of mutations, classified in this way, are complex. But the method provides reliable, sufficiently reproducible values of the mutation effects usable for comparison of mutagens or populations of organisms in the responses to mutagens under comparable conditions.