Some sources of vanauon in milk fat fatty acid composrtions of monozygous twin cattle housed indoors are described. Differences in fatty acid proportions between night and morning mil kings accounted for less than 8% of total observed variation in all acids except butyric; differences between 3 consecutive days for up to 21% of the variance in some acids; and differences between the six cows for up to 46% of the total variance. Significant (P < 0.05) differences between individual members of twin sets were observed in the proportions of most fatty acids.These sources of variation are discussed in relation to the design of short-term twin experiments where fat compositions are determined,