In the Netherlands Antilles two extreme “forms” of Artemia salina, f. arietina and f. milhauseni, show all intermediate gradations. They hybridize constantly. When forma arietina is moved from lower concentrations into highly concentrated brine it will produce offspring that exhibit characteristics of the milhauseni form. The forms, therefore, do not represent groups of any systematical importance. Growth rate and body size show great variation. Often, greatest growth rate and largest size are found at salinities between 45 and 200‰. Below and above that concentration growth is usually stunted, and mortality is high. Artemia is not capable of getting rid of adhering silt. Moreover, silt seems to prevent the uptake of food. Artemia is oviparous at low salinity concentrations, but from 85‰ upwards they become viviparous although viviparous females also produce some eggs. Differences in body colour are induced by the environment, but the colour of the ovary appears to be inherited. Adult Artemia prefer salinities from 50 to 150‰, but this preference is variable. Nauplii show a marked preference for freshwater and for more concentrated brines. This preference, however, appears to be only apparent, as the nauplii may have become trapped by the exigency of the environment. Artemia has a preference for light intensities that are somewhat less than that of a sunny day. Artemia prefers a temperature of 29 to 34°C. Interspecific food competition seems to be rare. There is an indication that rotatorians may be competitors. Intraspecific competition may occur, but no proof has been obtained. Poecilia sphenops and Cyprinodon dearborni prey upon Artemia; the fact that they are not specialized Artemia feeders is considered to be a disadvantage to Artemia. The salinity barrier for these two predators lies between 80 and 130‰, leaving the higher concentrations as a refuge to Artemia. However, Artemia will never attempt to escape from its predators. At lower salinities cyclopoids have been observed attacking Artemia. The absence of Artemia in water of low salinity can be attributed to predation. Omnivorous predators are more effective in this respect than carnivorous predators. A survey is given of the population fluctuations and of the factors involved.