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Gas chromatographic analyses of the nut oils of 155 samples, representing 15 taxa of Carya, confirm the basic uniformity of fatty‐acid composition within the walnut family. All taxa contain small amounts of the saturated fatty acids—palmitic and stearic—and variable amounts of the unsaturated oleic and linoleic acids, and at least trace amounts of linolenic. Quantitative differences in the relative amounts of the five fatty acids support the morphologic recognition of two sections within the genus. Members of the section Carya are characterized by higher linoleic and lower oleic acid percentages than are those of section Apocarya. It is also true, however, that tetraploids of section Carya tend to have higher linoleic and lower oleic acid percentages than diploids within the same section. Numerical analyses of the oil data reveal close similarities between certain members of the more “primitive” section Apocarya and the diploid members of section Carya. The highly heterogeneous assemblage within section Apocarya could be split into three groups on the basis of oil data. It is equally obvious, however, that the American taxa of the genus Carya have had a long and reticulate phylogeny and that recognition of additional categories above the species level would not result in natural assemblages.

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