The rotifers Brachionus plicatilis and Beta rotundiformis are used as live food for marine fingerlings. The mass production of this live food is still very unpredictabel, crashes (total mortality) regularly occur. The main objective of this project is the systematic study of possible geotypical aspects for the fluctuating character of the rotifer mass noduction. Brachionus strains of commercial hatcheries and strains of the ARC will be genetically characterized. The genetic diversity will be described and shifts in commercial culture can be followed.