Threespine sticklebacks (Gasterosteus aculeatus) breed in the shallows of lakes and streams during the spring when the males usually develop a greenish dorsum and a bright red color on the anterior undersides. The red is subject to sexual selection, and functions as a territorial threat display in the breeding colonies (Pelkwijk and Tinbergen, 1937; McPhail, 1969; Semler, 1971; herein). As we would expect, populations are frequently monomorphic for red males. McPhail (1969) discovered populations of sticklebacks on the Olympic Peninsula, Washington, where breeding males are entirely jet black (Fig. 1). He noted that their distribution coincides with that of Novumbra hubbsi, a small fish about the size of a stickleback which is endemic to a few drainages. We (Hagen and Moodie, 1979) have shown that there is a remarkably detailed correlation between the distributions of the two. Novumbra is now known to occur in four independent drainages, and in each, breeding male sticklebacks are black. Within the Chehalis River, western Washington, populations of sticklebacks are monomorphic red where Novumbra is absent and monomorphic black where it is present. In between there are steep clines in morph frequency with blacks increasing to 100% where they coexist with Novumbra. The red and black morphs are apparently controlled by two autosomal alleles without dominance, but their expression is affected by polygenic modifiers (Hagen and Moodie, 1979). McPhail (1969) showed with laboratory experiments that Novumbra: 1) is more successful at preying on the fry of red males than on blacks, and 2) shows a significantly greater orientation to red males that are tending their nests than to blacks. Thus, McPhail argued that as red males are more conspicuous while tending their nests, their offspring will suffer greater predation, so blacks will evolve where they coexist with Novumbra. The purpose of this paper is to give evidence supporting an alternative hypothesis. Cody (1974, and earlier) developed the hypothesis of convergence in threat displays among territorial species competing for resources that are at unusually low levels. If the available resources (or genetic variance in the species) do not allow the evolution of character displacement to avoid competition, convergence in threat displays may evolve. If the increase in fitness that results from excluding other territorial species using the resources exceeds the fitness of genotypes that do not (because they do not share a threat display), convergence for threat displays will evolve. Consequently, several species will use the resources as would a single species and the fitness of each is increased. Cody gives many examples of territorial species that converge in threat displays (visual or vocal) only where they are sympatric. Even more compelling are his examples of several territorial species that vary geographically in threat displays, and the coexisting species vary in parallel. We now know that, like sticklebacks, male Novumbra hold breeding territories where they are highly aggressive, and during reproduction they are predominantly black (Fig. 1). They breed in the same habitats with sticklebacks at about the