1. Cassiopea xamachana is unable to tolerate any wave action, turbulence or currents. 2. Although the species is found only in shallow water where the light intensity is high, it could not be demonstrated that light intensity is an important factor. 3. Normal temperature fluctuations — at least in the Antilles — are of no significance as the range that Cassiopea under laboratory conditions proved to be able to withstand is wider than the fluctuations occurring in their natural environment. On the other hand, after heavy rainfall when the pools are covered with fresh water the bottom water temperature may rise to deleterious levels. 4. The salinity range in the habitat of Cassiopea is from about 33‰ to 54‰. When the regeneration rate is used as a parameter for optimum salinity conditions, supersaline water of about 40‰ is optimal, which is about the average salinity in their natural habitat in Curaçao. In the Dry Tortugas, where the salinity is lower, optimum regeneration occurs at a lower salinity level, as reported by GOLDFARB, 1914.