The mucopolysaccharide capsules surrounding eggs of the Common frog, Rana temporaria temporaria L. were investigated. Frogs were obtained from ponds in northern England ranging in altitude from 46 to 838 m. Egg capsules acted as insulators, keeping the centre of an egg mass warmer on average than the surrounding water. The size of an egg capsule and its insulating efficiency varied in different pond waters. Egg capsules in water from highland ponds were larger and more efficient insulators than egg capsules in water from lowland ponds. In a series of laboratory experiments the factors controlling capsular swelling were investigated). The ionic concentration of the fluid surrounding the eggs was found to be the principal factor involved, but the temperature and pH of the fluid and the valency and identity of the ions in solution were also important. The water chemistry of ponds varied with altitude. This accounted for the differential swelling of egg jelly in highland and lowland ponds. The fact that egg capsules are larger in highland pond water may increase egg survival at low temperatures. This is probably fortuitous as there is no evidence to suggest that frogs select their breeding ponds because the water in them will promote capsular swelling.