Arbuscular mycorrhiza (MA) are mutualistic associations between plants and soil fungi. They can lead benefits for plants in the perspective of an improvement of soil fertility through a restoration of biological functionality, in particular by increasing phosphatic absorption, resistance to pests and pathogens, to water stress, and to salts. In the Italian SOSZOOT project, funded by Italian Ministry of Agriculture, the commercial product Micosat F® was used on maize crop and grains administered to lactating dairy cows. The aim of this study was to evaluate the effect of mycorrhized maize grain (MMG) on: milk yield, body weight, body condition score (BCS) and rumen environment of animals. The experiment was performed on 16 Holstein cows, divided in two groups homogeneous for parity, lactation stage, milk yield, milk quality, weight and BCS. The diet was isoenergetic and isoprotein (0.91 Milk FU/kg DM and 155 g/kg DM of crude protein). The only difference was the grain maize type (mycorrhized or not). The diets were administered for 105 days after three weeks of adaptation. Milk recording was carried on every two weeks. Body weight and BCS were recorded monthly. Besides, six cannulated heifers were fed the experimental diets in a cross over design. Rumen content (after 21 days of adaptation) was analyzed before morning feeding for pH and microbial population (protozoa and fungi, direct counts). Data were analyzed by GLM procedure (SAS, 1993). Milk yield was comparable between mychorrized and not mychorrized groups (30.40 and 30.90 kg/d respectively). In spite of a higher average daily gain (ADG) (+ 212 g/d, P