1. A description is given of form and components of the three types of spermatogonia: type A, I, and B. 2. The distribution of the stem cells (=type A-spermatogonia at the beginning of the cycle, i.e. after the release of the mature sperms) is rather irregular. 3. The mean number of stem cells per cross section of a tubule is the same in different parts of one testis and in both testes of one bull, but shows great differences between testes of various bulls (from 1.20 to 3.62 cells per cross section). Young bulls have the lowest numbers. 4. The number of the spermatogonia increases during the cycle by repeated mitoses. This multiplication takes more time than one cycle: 4 mitotic peaks occur in one cycle (stages 6, 10, 14, and 16), the fifth in the next cycle (stage 6 — at the same time with the first mitoses of the following multiplication process). Not all spermatogonia divide at each peak. This is one cause of the differences in multiplication results between different testes, degenerations being a second cause. The mean number of primary spermatocytes arising from one stem cell varies in different testes from 8.3 to 17.8. 5. The type A-cells become type I-cells after mitotic peak 2 (stage 10), change into type B-cells after peak 3 (stage 14) and become primary spermatocytes during peak 1 of the next cycle (stage 6). A number of type A-cells shirks the mitotic peaks 3 and 4. They are resting cells till peak 1 of the next cycle, being the stem cells of the next multiplication process. 6. The mitotic peaks are compared with those of other animals. 7. Correlations between mitoses and other events in the spermatogenetic epithelium exist. The correlation between mitotic peak 3 and the completion of meioses appeared to be very close. Sertoli cells must be held mainly responsible for these correlations. 8. It appeared to be impossible to give a scheme of the multiplication of bull spermatogonia, the degree of freedom to divide being too great.