We studied leaf fluctuating asymmetry in Myrtus communis occurring around the “I Borboi” natural CO₂ spring (Tuscany, Italy) and in a nearby control site, in a Mediterranean environment. Developmental instability, measured as leaf fluctuating asymmetry, is expected to be positively related to environmental stress and negatively to habitat quality. A gradiental decline in leaf width and leaf angle fluctuating asymmetry was found along the decreasing CO₂ concentration from double to ambient. The correlation is ascribed to the positive effect of elevated CO₂ on developmental stability. Probably, the adaptation process of individuals grown in close proximity to the CO₂-enriched area took advantage of improved water use and carbon balance of M. communis plants.