The research was aimed at comparing contents of cis9trans11 C18:2 acid (CLA) and trans isomers of C18:1 acid in commercial yoghurts and bioyoghurts as well as acquiring information whether CLA concentration in yoghurts and bioyoghurts differs from that in the initial material. Determinations were carried out with the method of gas chromatography using a 100-m capillary column with CP Sil 88 phase. Investigations demonstrated that in commercial yoghurts and bioyoghurts analysed the content of cis9trans11 C18:2 acid (CLA) and the total content of trans isomers of C18:1 acid were alike. The mean content of CLA in fat isolated from the yoghurts examined ranged from 0.41% to 0.43% of the total fatty acid composition, whereas in fat extracted from the bioyoghurts under study it ranged from 0.41% to 0.44%. The total content of trans isomers of C18:1 acid in yoghurts oscillated between 1.66% and 2.34% of the total fatty acid composition, and that in the bioyoghurts – between 1.95% and 2.91%. The content of cis9trans11 C18:2 acid in the total fatty acid composition of fat obtained from raw bulk milk examined did not differ from its content in fermented drinks produced from that milk. The mean content of CLA in fat of the yoghurts accounted for 0.53%, whereas that in fat of milk they were produced from – for 0.52%. Fat of the bioyoghurts analysed was characterised by a slightly lower content of cis9trans11 C18:2 acid as compared to the raw material. The mean content of trans isomers of C18:1 acid reached 2.25% in yoghurts, 2.00% in bioyoghurts and 2.13% of the total fatty acid composition in raw milk.