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The systematic difference in the c^b -y) diagram between the sequence of unevolved stars in the Hyades and the corresponding sequence for unevolved field stars with the same metal abundance is discussed. Within the framework of current models for stellar interiors and atmospheres the difference is interpreted as a helium abundance difference, such that the helium content of the Hyades is lower by 0.06 in n He /n H , as compared with the corresponding field dwarfs. Other possible explanations for this difference are discussed but found to be less probable.The corresponding difference in the c^b -y) diagram for the stars in the Hyades as compared with the Coma cluster stars is interpreted as a similar difference in the helium abundance.

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