The sex ratio (percentage of male) of offspring in swine was examined on the basis of the farrowing records obtained from the St. Isidore Farm in Cheju island. The records were available for 11,260 pigs from 612 litters of Landraces (5,798 pigs) and 619 litters of crossbreds (5,462 pigs) farrowed from 1968 to 1971. The results obtained in this study are as follows. 1. The sex ratio of 11,260 pigs was 52.94%, 54.27% for the Landrace and 51.52% for the crossbred. The sex ratios were significantly different from 50%. No significant difference was observed between the two breeds, although the sex ratio tended to be higher in the Lardrace than in the crossbreds. 2. The sex ratios for the years 1968 through 1971 were 51.82%, 50.92%, 51.55%, and 53.36%, respectively. The sex ratio for 1971 was significantly different from 50%, but those for other years were statistically insignificant. The Landrace sex ratio was especially high in 1970 and 1971. The sex ratio difference between years was not significant. 3. The sex ratio was 49.00% far spring, 54.51% for summer, 51.00% for autumn, and 51.05% for winter. The sex ratio for summer was significantly different from 50%, but they were statistically insignificant for other seasons. The sex ratio for summer was significantly higher than that for spring and autumn, and the sex ratio for autumn was significantly higher than that for spring. 4. The sex ratio for the first, second and over third parity was different from 50%. There were no significant differences among parities. 5. The sex ratio was not significantly different from 50 0, for most litter sizes, but was significant for the litter sizes of 6, 7, 8, 9, and 13, and for the 6-10 group. No significant differences were observed between litter sizes. 6. The sex ratio was significantly different from 50% only in 4 Landrace sires, but the sex ratio differences among 14 sires, 8 Landrace, 3 Large White, and 3 Duroc sires, were statistically insignificant.