Nine samples of natural yogurts, their fat content varying (from 1, 0 to 4, 3 %), were tested. For the purpose of determining a viscosity and shear stress, samples of five types of yogurts having different expire dates were investigated during a 240 s long shearing testing procedures, at the constant shearing rates of: 5, 10,15, 20, 30, 50, 70, 100, 150, 200, 300, and 500 s^-1. Another four types of yogurts were investigated in a similar way; each sample was tested during a period of 723 s at the constant shearing rates of 30, 50, 70, 100, and 150 s^-1. Some rheological parameters were measured; then, a trial was made to describe those parameters using rheological models. The investigation results were compared with the emulsion structure of the medium investigated. On the  basis of experiments performed it was stated that there was a relationship between the values of rheological parameters and the ageing time of yoghurt, as well as a relationship between the rheological parameters and the emulsion structure of the medium tested. The experimental data were better adjusted using a Herschel-Bulkley model than an Ostwald-de Waele model.