The stability of morphological features, traditionally used for the infraspecific classification of Thymus pulegioides L., was studied. The individual plants of T. pulegioides were collected from different habitats and cultivated under the same field conditions by cloning them annually for five years. The results of the cultivation experiment indicated that most of the investigated morphological features including length of leaves, length/width ratio of leaves and length of inflorescences were unstable. A conclusion was approached that the natural variation of the investigated morphological characters of T. pulegioides is not genetically fixed. It is phenotypic plasticity of plants, which depends on environmental conditions. Therefore the morphological characters, used at the “traditional” infraspecific classifications of T. pulegioides, cannot be used as the reliable diagnostic indicators of the infraspecific taxa of the species.