The bottom sediments of the southern part of Pyramid Lake consist of fine ashy sands near the mouth of Truckee River and of diatomaceous, ostracodal, ashy, copropelic, and sapropelic silt away from the river month. The wet sediments are somewhat less alkaline and have stronger reducing intensity than the overlying waters. Bituminous extracts of the sediments show the presence of probable hydrocarbons, and are characterized by a fairly high content of pigmenting substances. The quantities of amino acids and humic substances in Pyramid Lake sediments are reminiscent of oligotrophic lakes in the north central part of the United States. The term "apatotrophic" is introduced for lakes of arid regions which have alkaline waters and high total dissolved solids but relatively low C and N content, and in which the sedimentary bitumens are characterized by relatively large amounts of pigmented organic substances.