The size dependency of maximum growth rates was investigated in cyanobacteria and in green algae (Chlorophyta). Both unicellular and colony-forming species were included in the study. Significant allometric relationships were found between size and maximum growth rate for both cyanobacteria and green algae. The size-dependent growth could be described by the same scaling exponent in both cyanobacteria and green algae, but in both cyanobacteria and green algae only unicells evinced size-dependent growth rates—there was no relationship between colony size and growth rate in colonial forms of cyanobacteria and green algae. It is concluded that the colonial growth form represents an evolutionary adaptation to escape the negative effects of size-dependent growth, while retaining the positive effects of increased size, e.g. a decreased grazing pressure.