Colonial growth by blastogenetic budding in colonies ofBotryllus schlosserihas been studied in field and laboratory. It can be expressed either by the number of individuals making up the colony at the various blastogenetic generations or through growth rates defined as rn-1= Xn/xn-1= growth rate at generation (n — 1)th, where xnis the number of adult zooids of the n — th generation and xn-1that of generation (n — 1)th. This second method seems to be better.