Dry matter production of orchardgrass swards was investigated and some analysis of its growth was made in 1967 and 1968. The effects of two levels of cutting height on growth were compared under two levels of nitrogen fertilizer treatments. This paper deals mainly with the overall response of the sward to the seasons, and the results relating to these treatments will be described in detail in the following papers. Plants were harvested at 5 to 10 days interval for a period of 6 to 10 weeks in early spring, spring, summer and autumn. Leaf area and dry weights of leaf blade, leaf sheath, stem, root and litter were measured. Dry matter yield per year was 720g/m^2. Dry matter yield per day (Y) was the highest in the early spring and the lowest in the autumn. Net production per day (P_n) was the highest in the early spring and the lowest in the summer. The seasonal change of P_n corresponded with that of net assimilation rate (NAR). Y/P_n ratio was 40-50, 71 and 21% in spring, summer and autumn, respectively. It was considered that Y/P_n ratio was of importance in determining the yield. It was indicated that the low yield in summer was associated with the low P_n caused by high temperature, etc., and that the low yield in autumn was associated with the low Y/P_n ratio. Root dry weight decreased more sharply and remained small for longer duration after cutting and recovered slower in summer than in the other seasons. On the contrary, in autumn, root weight did not decrease much and increased again from the early stage after cutting. In autumn, plants had higher C/F ratio than in spring and summer. The growth process after cutting was classified into three stages, 1) the stage in which leaf sheath, stem and root weight decreased, 2) the stage in which leaf sheath and stem weight increased gradually, and 3) the stage in which root weight increased quickly. Leaf area indices (LAI) reached to 5.5 to 6.0 at about 30 days after cutting in spring, summer and autumn, showing not much difference among those seasons. But in summer, 1967, LAI was only 3, presumably leaf growth was inhibited by high temperature and drought. Leaf area duration had high positive correlation with P_n except in summer, when P_n was low in proportion to leaf area duration. The growth was characterized by rapid shoot growth and high P_n in early spring, relatively high P_n in spring, low NAR and P_n in summer, and vigorous root growth and high C/F ratio in autumn.