Asellus aquaticus shows continuous variation in phenotype from white through yellow and brown to black. An artificial polymorphism was created by choosing the palest and darkest individuals, and these showed no preference for their background when offered a choice of black or white substrate. Sticklebacks exhibited no differential predation when the two phenotypes were offered to them at equal frequency. However, when phenotypes were offered in frequencies of 4:1, the sticklebacks chose an excess of the commoner form whether dark or light. The use of the stickleback-Asellus system in analysing density-dependent and frequency-dependent predation is discussed.