The author has studied the relative growth of the body weight, the lengths of tail, ear and hind foot to the length of head and body of the Japanese harvest mouse, Micromys japonicns (Thomas). The materials used in this study are those which have been collected in the campus of Kyushu University, Hakozaki, Fukuoka and the dry-river bed of Chikugo River in the suburbs of Kurume City, Fukuoka Prefecture, using snap traps and cage traps. They are composed of 88 males and 71 females. These places are rich in miscanthus and offer good habitats to this kind of mouse. The period of collection was from April, 1957 to March, 1961. Measurements were restricted to such external characters as body weight, tail, ear and hind foot length, because being readily measured. The baby young that were seemed at the age of one or two days old (hairless and reddish with closed eyes) had the length of head and body of 28 to 29 mm. From the above fact, classification of materials was made at 5 mm intervals starting from head and body length 25 mm. The mean value secured in each class was made x and those of other four characters were made y respectively. By plotting x and y on the double logarithmic graph, the author could acquire two or three straight lines which differ in their gradients. From the above truth, it is proved that J. S. Huxley's relative growth formula y=bx^α can be applied. By comparing the relative growth coefficients, the author has tried to analyze the growth pattern of this kind of mouse. Summaries are as follows 1) The growth of body weight is divided in t o three phases. The first period coincides with 25 to 49 mm H.B. (in both sexes). The value of a of male is 3.39 and that of female is 3.42. The second period ranges from 49 to 64 mm H.B. and a of male is 1.18 and that of female is 0.97. Thus the second growth stage is isometric one. The third period is over 64 mm ILB. and a of male is 2.97, while 2.58 in female. 2) Tail grows at two different rates. The first stage ranges from 25 to 46 or 48 mm H.B.. The value of a is 3.09 in male and 2.75 in female. The second a is 0.72 in both sexes. Tail coefficient is the largest at the point where the first period changes to the second (1.266 in male and 1.241 in female). Beyond this head and body length, this value decreases gradually, but is always larger than 1.00. 3) Growth of ears of both sexes shows two different phases. The inflexible point is at 48 mm H.B. in both sexes. The value of a in the first phase is 2.62 in male and 3.12 in female, while that in the second phase is 0.19 in male and 0.25 in female. This fact indicates that the growth of ear decreases relatively after the length of head and body becomes larger than that at the inflexible point. 4) The growth of hind foot length is divided into two phases. The inflexible point is at 47 to 48 mm H.B.. The value of a of male is 1.86 and that of female is 1.65 in the first stage, while 0.22 in male and 0.14 in female in the second stage. 5) A s a whole, the inflexible point at which the growth phase changes from the first to the second coincides with the independence of this kind of mouse from its mother's nursing. But the exact relation of its age, weaning and sex maturity to each phase is obscure. This problem must be left to be clarified later.
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