Observations on populations of Branchinecta mackini in temporary ponds, which last for as little as 3 days to as long as 4 months, on a desert playa showed that hatching of dehydrated cysts (eggs) follows initial entry of water into the basin if salinity remains low. Salinity ranges from an initial 0.5% to as much as 34% as the pond evaporates. Hatching is continuous at constant low salinity, but since salinity general increases rapidly, initial hatch is usually of short duration. Additional periods of hatching follow further inflows of water or after meeting of ice, that is, after reductions in salinity. The duration of hatching is inversely proportional to rate on increase in salinity. When salinity of small—volume summer ponds increases at rates above 500 ppm (1,000 (mu)mhos) per day, three in virtual inhibition of hatching. Laboratory studies showed that egg hatching was controlled by both salinity and oxygen operating in various combinations to inhibit or stimulate hatch. The hatching characteristics obtained from laboratory cultures after ejection from ovisacs of living females. The finding that a salinity—oxygen complex regulates hatching in a desert pond permits tentative explanation of a difference between branchiopods of humid and arid regions. In both cases the branchiopods are characteristic of astatic waters, and stimulation of the egg to hatch must be by some factor that changes at the time or origin of the temporary pond. In humid regions the factors of concern are temperature and oxygen. If the water undergoes significant changes in salinity, as it does in arid regions, control of egg hatch may be by both salinity and oxygen, with temperature limited to control of rate of development.