Arcturus Lake is a stratified hypersaline lake occupying a caldera near the center of Genovesa Island. It is about 500 m in diameter and has a maximum depth of 30 m. The lake is frequented by large numbers of seabirds and is an outstanding example of guanotrophy. Organic production in the epilimnion was great, and the hypolimnion was anoxic at the time of these studies. The lake is probably stratified during the entire year. There appear to be no fishes in the lake. The invertebrate fauna is dominated by corixids and tanaid shrimps. Tagus Crater Lake on Isabele Island, occupies a crater about 100 m from Tagus Cove. It is about 400 m in diameter and 10 m deep over most of the basin. The lake is saturated salt brine with a bottom of rock salt. The fauna is dominated by corixids and anostracan shrimps. Of three small ponds at Noah's Coves. Isabele, one has low salinity from top to bottom, whereas the other two are markedly stratified with regard to salinity. The upper "fresher" water in the stratified ponds is poor in phytoplankton compared to the underlying high—salinity water. Several species of fish, Odonata, corixids, water striders, and crustacean larvae were living in the ponds at the time of these observations.