Phosphorus deficiency is frequently the limiting factor in phytoplankton growth and the major source of selective pressure on single populations in a variety of aquatic environments. Phytoplankton species have several mechanisms that allow them to overcome phosphorus starvation, such as high-affinity uptake of orthophosphate (P-i), stand-by energy metabolism or utilization of phosphorus from stored polyphosphates. This short review is focused on the production of extracellular phosphatases that is usually accompanied by high-affinity uptake of P-i. Extracellular phosphatases are localized on the cell membrane, where they catalyze the liberation of P-i from various organic compounds that are too large to be directly transported into the cell. The production of extracellular phosphatases is not a general response to starvation in phytoplankton; it is a species-specific or cell-specific feature. The ELF (Enzyme Labelled Fluorescence) technique enables direct visualization of the sites of enzymatic activity and therefore gives detailed information about the physiological status of natural phytoplankton.