Three groups of independently conducted tests demonstrate selective predation on sticklebacks having various plate numbers. Differential survival of sticklebacks in some tests was clearly due to behavioural differences. Since fish from two populations were used, however, we are unable to say whether this indicates differential predation upon plate classes, or differences between populations, or both. Fish with seven plates seemed to be at a selective advantage only under winter conditions in the laboratory, and these results conflict with those from the field where sevens are at an advantage during the summer. This may result from behavioural changes associated with breeding during the summer. Results of test conducted under winter conditions are in accord with selective predation observed in three natural populations. Given the different feeding behaviour of the predators it seems likely that the traits contributing to survival are geneeral ones that function in a variety of physical and biotic environments.