Introduction. Article presents evaluation of quality of Canadian commercial plain non-fat Greek-style yogurts produced only from natural dairy ingredients. Material and methods. Yogurts samples were purchased 10 days after their production and were stored at 5oC to obtain 18 or 35 days of storage. Dynamic rheological properties and surface wheyoff were measured. An ordinal numerical scale was used in order to evaluate whey drainage and the presence and size of visible clusters. Results and discussion. Greek-style yogurt is a typical weak viscoelastic gel whose elastic properties predominate over its viscous properties over the measured range. A structural degradation was observed in all samples at some point during the stress-amplitude range applied. B samples had a higher total solids content than A samples therefore, this might be the reason that B samples presented higher dynamic moduli than A samples, even for the same protein content. Both reference samples presented a significant increase in their viscous (G") and elastic (G') moduli upon storage at 5oC. This fact suggests that casein gels are dynamic by nature and that further development of the gel structure occurs during storage. At high amplitudes, A samples presented a significant increase in their tan  values due to the rupture of their gel structures. A proportional increase in G' and G" during storage was observed; hence, the tan  values for the same types of samples, after 18 and 35 days of storage, were similar. Samples did not differ in levels of whey drainage and in the size of visible clusters. Both market samples presented higher amounts of surface whey-off as storage time increased. Probably during storage, large scale rearrangements occurred in the gel network which increased the level of instability of the gel, resulting in the loss of the ability to entrap all the serum phase. Although none of the reference samples had visible whey drainage, all of them presented small visible clusters. Conclusions. Protein is not the only ingredient shaping structure and rheological properties of Greek-style non-fat yoghurt. Properties of commercial plain non-fat Greek-style yogurts changes at the storage time. Samples becomes more solid, but with higher whey syneresis.