The hydrocarbon contents of 23 species of algae (22 marine planktonic), belonging to 9 algal classes, were analyzed. The highly unsaturated 3,6,9,12,15,18-heneicosalhexaene predominates in the Bacillariophyceae, Dinophyceae, Cryptophyceae, Haptophyceae and Euglenophyceae. Rhizosolenia setigera contains n-heneicosane, presumably derived from the hexaolefin by hydrogenation. Two isomeric heptadecenes have been isolated: the double bond is located in 5-position in the bluegreen alga Synechococcus bacillaris and in 7-position in 2 green algae. Our complete analyses are discussed in the context of earlier data; some generalizations appear no longer valid. Hydrocarbon analysis of marine algae should provide a tool for the investigation of the dynamics of the marine food chain. Knowledge now available provides the background needed for distinguishing between hydrocarbons of recent biogenic origin and hydrocarbon pollutants from fossil fuels.