Thais (or Nucella) emarginata, a rocky intertidal gastropod inhabiting the Pacific coast of North America, is polymorphic for shell banding in nearly all habitats in which it is found. Two generations of breeding revealed that this polymorphism has a simple genetic basis. Banding of the outer shell was inherited as if controlled by a single autosomal locus (OB) with two alleles (OBB = banded and OBU = unbanded), with banding dominant. Banding also assorted independently of the major shell color locus (OC), thus reinforcing an earlier conclusion that shell traits in this species are not tightly linked in a `super gene' as in many terrestrial pulmonates. The clarity of banding, however, was found to depend on other genes or alleles influencing pigment intensity; individuals carrying these alleles at the banding or other loci exhibited much less pronounced banding or in some cases a complete loss of pigment in the outer shell. Most commonly, spiral bands appear as regularly spaced lines of pigment set against a largely unpigmented background (= white to pale grey ground color), but bands may also occur against other ground colors. Mechanistically, however, banding appears to result from regularly spaced zones of suppressed pigmentation in the outer shell. Preliminary distributional data revealed that the frequency of banded individuals in field populations increased with increasing wave exposure; however, the adaptive value of this polymorphism is not clear at present.