Perennial swarms of sexually immature medusae of the germs Mastigias occur at high densities in three marine lakes in Palau, Western Caroline Islands. Numbers per lake are 1.76 ± 0.10 × 10⁵, 1.61 ± 0.17 × 10⁶, and 4.69 ± 0.51 × 10⁶. Local density within a swarm may exceed 1,000·m^‒2. Behaviorally generated swarms form in the early morning and late afternoon and are temporarily dispersed by directional horizontal migration of the entire swarm twice a day to opposite ends of the lake. Distance swum each day is up to 1 km, depending on the size of the lake. Compass direction of migration is different in each lake and appears to depend on both light and basin morphometry.