Farnesal and 3-hydroxy-2,3-dihydrofarnesal (3-hydroxy-3,7,11-trimethyl-6,10-dodecadiene-1-al) were formed from farnesol when the alcohol was incubated with the protoplast of Botryococcus braunii B race strain. This fact suggests the existence of farnesal hydratase in the alga. Feeding experiments showed that both farnesal and 3-hydroxy-2,3-dihydrofarnesal were efficiently incorporated into botryococcenes, triterpenoid hydrocarbons of the alga.