RYEGRASS (POACEAE) COMPRISES 6 SPECIES IN DIFFERENT HABITATS OF IRAN. THIS STUDY WAS CONDUCTED IN MAZANDARAN PROVINCE DURING 2007-2008 ON lolium perenne WEED. GEOGRAPHICAL DISTRIBUTION, CO- OCCURRING SPECIES, PHENOLOGY, PHYSICOCHEMICAL SOIL PARAMETERS, CLIMATICAL STUDIES AND OTHER EFFECTIVE FACTORS ON DISTRIBUTION OF THIS SPECIES INCLUDING ALTITUDINAL MODIFICATIONS WERE EVALUATED. STUDY WAS PERFORMED IN 9 ECOLOGICAL SITES IN MAZANDARAN PROVINCE. RESULTS SHOWED THAT L. perenne WAS PRESENT IN ALL SITES AND IS FOUND FROM -22 M. (BABOLSAR) TO 1700 M (GALOUGAH) ABOVE SEA LEVEL. THE SPECIES IS MORE ABOUNDENT IN LOWLANDS THAN HIGHLANDS. THE PLANT WAS AN ELEMENT OF HUMID TO SUB HUMID CLIMATE AND ALPINE AND COLD MEDITERRANEAN HABITATS. SOME OF MOST IMPORTANT CO-OCCURRING SPECIES IN MOST SITES WERE: AEGILOPS CRASSA, HYPERIUM PERFORATUM, MEDICAGO LUPULINA, POA ANNUA, CYNODON DACTYLON, TRIFOLIUM REPENS, ARTEMISIA ANNUA. THE INITIATION OF VEGETATIVE SATGE, SPIKE FORMATION, FLOWERING AND SEED FORMATION WERE DETERMINED IN PHENOLOGICAL STUDIES FOR BOTH LOWLAND AND HIGHLAND SITES.