Identifying the population origins of individual fish is important in assisting the policing of fishing waters and the tracking down of poachers. In the September 20 Nature, Einar Nielsen and colleagues from the Danish Institute for Fisheries Research describe a simple approach using microsatellite markers to assign individual Atlantic cod fish (Gadas morhua) to their original population (Nature 2001, 413:272). They studied three cod populations; from the North Sea, the Baltic Sea, and the northeastern Arctic Ocean. They analysed nine microsatellite markers and found strong genetic differentiation between the populations. Using a rigorous statistical procedure, they were able to assign individual fish to one of the three populations with high reliability, independent of considerations of migration rates. The application of this methodology could help in the conservation of important fish species in heavily trawled waters.