Spotted dolphin ( Stenella attenuate, Gray, 1846) is a kind of small cetaceans, and widely distributes along the east and west coastal waters of Pacific Ocean. Spotted dolphins have been capturing in Japanese coastal waters for a long time. Specimen of 286 spotted dolphins used in this study were collected in Taiji, Japan from 1991-1994, which were captured by driving fishery method there. Problems related to age determination, growth and some population parameters of the dolphin were analyzed in this study.Tooth of the dolphin was sliced lognitudinally with 15 mu m in thikness, and stained with improved Mayer's hematoxylin. The so called improved Mayer's hematoxylin is that concentration of the normal Mayer's hematoxylin is doubled here (1 g/1000m 1). By comparison, staining effect by using the improved Mayer's hematoxylin is better than that by using the normal Mayer's hematoxylin.A lognitudinal slice of a tooth looks like a leaf of willow under microscope with transmitted light (Figure 3). It can be clearly seen that a tooth is divided into two parts by a high-lighted neonatal line. The part inside of the neonatal line is dentine, and the out side is cementum. Layers can be found in both dentine and cementum. Growth Layer Groups (GLGs) in dentine are different from that in cementum. One GLG in dentine could be divided into four sub-layers under microscope with transmitted light, which are marked sub-layer A, B, C, and D respectively in this study (Figure 1). One GLG is defined equaling to one year. Usually, the first 2-3 GLGs near the neonatal line looks wider because the animal grew faster in younger ages. GLGs in cementum usually consist of two sub-layers, one bright and one dark in transmitted light. Likewise, one GLG in cementum is also defined equaling to one year (Figure 2). Excepte for clear GLGs can been seen in transmitted light, some fine layers could be found within sub-layers in dentine in some individuals. Some of the fine layers could be counted as many as 12-15 layer in one GLG, which could be thought of as a monthly growth rhythm. These fine layers are tentatively defined as Lunar Monthly Layers (LMLs) here. Further more, 5 to 6 finer layers were founded in the fine layers in some individuals (Figure 4). Each of the finer layers might represent about one week.Comparing accumulating rates of GLGs in dentine and cementum, it could be found that the rates are nearly same before age 5. They become different from age 6 through 9 gradually. Along with the age increasing, the rates vary incrementally during age 10 and 18. Generally, the number of GLGs in dentine is smaller than that in cementum since pulp cavity closes after age 18, the GLGs in dentine stop increasing, while the GLGs in cementum continue to increase. The maximum counting of GLGs is 18 in dentine and 42 in cementum (Figure 5).The relationship between age and body length, in a certain phase I follows the model of power function (Y = aX(b)). A rapid growth of body length, in both sexs, appears at the age younger than 5-6 years and reduces gradually later on. Between 8-14 years, the males grow up faster than the females. It was shown from age composition (Figure 7) that proportion of younger, under 9 years, was relatively low, especially the males about 5 years. Asymptotic length of males was 201cm at age of about 22 years and females was 191 cm at age of about 17 years (Figure 8).