A column chromatographic technique, enabling identification and quantitative estimation of fatty acids, has been employed to study fat metabolism in Citrullus during germination in the light. This plant is characterized by an unusually rapid disappearance of storage fat as the cotyledons expand and turn green. In spite of the high catabolic activity there is no evidence for accumulation of free fatty acids or short-chain fatty acids at this stage. Information on this point derived from acid value or saponification value of the oil is shown to be untrustworthy. Citrullus seed fat contains the following percentages of acids: linoleic 70·6, oleic 7·2, palmitic 10·1, stearic 11·2, and arachidic 0·6, and careful analysis has also revealed small amounts of octadecatrienoic acids, both conjugated and non-conjugated. All the major acids are broken down at rates proportionate to the quantities originally present, with the exception of oleic acid which is metabolized somewhat more rapidly. ‘Linolenic’ acid is synthesized in the expanding green cotyledons and the fatty acid composition of the latter, in the late germination stages, resembles that of a green leaf and is very different from that of the seed. The results suggest a rapid removal of storage fat from the cotyledons and concomitant formation in small quantity of a typial leaf fat as the new photo-synthetic function develops.