A seeding rate for oat (Avena sativa L.) of 500 viable seeds m - 2 is used in Finland for development of a uniculm stand, and to avoid post-anthesis tillering and subsequent uneven ripening of the crop. Plant breeding has, however, considerably altered plant height and plant stand structure, thereby possibly changing tillering capacity. The objective of this study was to assess the contribution of tillers to grain yield and morpho-physiological traits comparing an oat landrace cultivar (Jalostettu maatiainen), a modern cultivar (Puhti), and a semi-dwarf line (Hja 76416) when grown at 200 to 900 viable seeds m - 2 at increments of 100 seeds m - 2 . The experiments were conducted at Viikki Experimental Farm, University of Helsinki, Finland (60°13 N), in 1991 and 1992. Increasing the seeding rate significantly increased the number of main culms m - 2 , but decreased straw length and several yield components on the main shoot. Tillers in the semi-dwarf line contributed 27% to the grain yield at 200 seeds m - 2 . This line yielded highest at 600 seeds m - 2 which favored a uniculm growth habit, and 98% of grain yield was solely from main panicles. Due to the sensitivity of the landrace and modern cultivar to lodging, no significant effect of seeding rate on grain yield was recorded under favorable growing conditions (1991). The highest grain yield for them was recorded at 600 to 800 seeds m - 2 with early summer drought (1992). No tiller contribution to grain yield was observed at 500 seeds m - 2 . Following sensitivity to lodging the optimum seeding rate for the longer strawed lines is lower and more dependent on environmental factors than that for the semi-dwarf line.