The repeating sequence of isopycnic twin Satellite II of the hermit crab, Pagurus pollicaris, has been determined. More than 95% of the primary sequence of the light strand is composed of (CAG)nCTGCACT. The only detectable variation on the repeating sequence is the number of CAGs included in each repeat which varies from 3-12. The sequence of Satellite II differs markedly from that of its twin, whose light strand is greater than 95% CCTA (Skinner et al., 1974). The absence from Satellite II of repeats where (CAG)n less than or equal to 2 further defines the constraints on a recent model for the origin of satellites (Smith, 1976).