Enhanced biological phosphorus removal activated sludge plants often do not remove phosphorus adequately in order to meet legal demands. Currently FeS04 is being added to almost all South African nutrient removal activated sludge systems discharging effluents to the sensitive catchments to prevent phosphorus from entering fresh water systems. In order to understand biological phosphorus removal mechanisms in order to optimise the process, the role of growth rate and phosphorus removal in Acinetobacter was investigated. Phosphorus was accumulated in the lag phase of the normal growth cycle. Little or no phosphorus was accumulated in the logarithmic growth phase, instead phosphorus was released at the beginning of logarithmic growth. Further phosphorus accumulation took place in the stationary phase, once active growth had ceased. Cells had a limit to the amount of phosphorus that could be accumulated per cell irrespective of substrate availability. It was therefore concluded, that the number of cells (biomass) in a system and their growth stage were crucial factors governing biological phosphorus removal. Maximum cell numbers should therefore be obtained and logarithmic growth should be prevented in the aerobic zone, in order to optimize biological phosphorus removal from activated sludge.