Topsin M is a common systemic fungicide used as protective/curative substance for alimentary and medicinal plants. That’s why it’s relevant to evaluate the influence of Topsin M upon the morphological features of Mentha longifolia, a volatile oil producing medicinal plant and parental species for the hybrid Mentha × piperita. We noticed statistical significant variations (Oneway Anova) of the dimensional features of the leaves from Topsin M treated plants comparing to the control.