The viability of yoghurt bacteria and two commercial strains of bifidobacteria was assessed in either yoghurt containing chicory fructooligosaccharide (FOS) or without any prebiotic, during 28 days storage at 4 degreesC. All the products showed a decrease in the viable count of yoghurt bacteria and bifidobacteria during storage. Numbers of Lactobacillus delbrueckii ssp. bulgaricus decreased faster than those for Streptococcus thermophilus. The viability of bifidobacteria in yoghurt was affected by the strain type and the presence of FOS. Bifidobacterium animalis exhibited better stability in the yoghurt than B. longum. The recommended level of I million cells was exceeded for B. animalis throughout storage. The highest viable number of bifidobacteria (3.59-2.25 x 10(7) CFU g(-1)) was obtained in the product containing B. animalis and FOS. Viability of B. longum in yoghurt containing FOS remained above 10(6) CFU g(-1) for up to 21 days, whereas this level was maintained for only 7 days for that organism in yoghurt without any prebiotic.