Crop load regulation in modern apple fruit growing is an essential tool for obtaining regular and high quality yields. Common applied thinning products in Italy, such as naphthaleneacetic acid (NAA), naphtaleneacetamide (NAD), benzyladenine (BA), 2-chloroethylphosphonic acid (ethephon) and ammonium thiosulphate (ATS), do not always give satisfactory and consistent thinning results. Therefore, the interest farmers have in finding new thinning agents remains high. Within the present study, the impact of metamitron as a thinning compound was tested on 'Golden Delicious' CloneB Lb®, 'Golden Delicious' Reinders® and 'Fuji' KIKU®8 apple plants, in different open field trials. Results indicate that metamitron reduced fruit set in a concentration-dependent manner and highest abscission rates were encountered when thinning was conducted at 12 mm fruit diameter. Fruit drop increased generally in those plants, where leaf net photosynthesis was strongly inhibited by metamitron. At the same time, an increase in leaf chlorophyll fluorescence of photosystem I was detected. Maximum photosynthetic stress levels were recorded approximately 6 days after application, whereas a complete recovering occurred within 3-4 weeks. Fruit quality parameters such as fruit size, fruit color, soluble solid content, and titratable acidity were improved by metamitron. Fruit russeting remained unaltered, besides some leaf phytotoxic effects compared to treated plants. So far, only the poor return bloom remains a concern. To sum up, the present study shows that metamitron would represent a very promising and powerful tool for regulating apple crop load.