Certain species of Brassica so far investigated will be classified into three groups, according to the behavior found for incompatible pollen and pollen-tubes, as follows: 1) Some incompatible pollen grains do not germinate and others produce very short tubes (STOUT in Brassica pekinensis, and SEARS in Brassica oleracea var. italica). 2) Pollen germination is normal, but pollen-tubes are inhibited in the style (KAKIZAKI in Brassica oleracea var. capitata). 3) Pollen-tubes grow normally, and reach and fertilize the ovules(SASAOKA in Brassicaoleracea var. capitata). The present investigation consists mainly of histological observations of self-incompati-Bility in Brassica chinensis, B. japonica, and B. japonica var. indivisa. Self- and cross- pollinations were made on the same plants respectively. In these experiments the pistils were collected at intervals, killed in acetic alcohol, slit with a razor blade lengthwise into three sections and stained with 0.5 percent lactic-blue for about an hour. After being differentiated in lactic acid for about 24 hours, mounts were made in a drop of glycerine. Sufficient pressure was applied to the cover glass to squeeze the pistil into a thin layer. The results obtained in 1940 and 1941 may be summarized as follows: Brassica chinensis LINN. A few pollen grains were found on the stigma after self-pollination. It is thought that most of the pollen grains which failed to germinate had been washed off in the treatment of fixing and staining. The germination of incompatible pollen was poor. Although many stigmas were examined, only one pollen grain was found which had emptied its contents into its tube. Pollen-tube length usually wa? about as long as the diameter of a pollen grain. Nearly all the tubes did not penetrate into the stigma. Some tube ends were characteristically coiled around the papillae as in Brassica pekinensis(STOUT 1931). The cross-pollinations were decidedly superior in the total number of pollen grains on the stigma and in the percentage of pollen germination to the self-pollinations. Numerous emptied pollen grains were found The tissue of the stigma was penetrated freely by pollen-tubes. The longest tubes in many styles examined could be found in the ovary in about 6 or 8 hours after pollination. Brassica japonica SIEBOLD Incompatible pollen was affected in the same way as inBrassica chinensis, except for the fact that only a few emptied grains and tubes of cosiderable length were found. Although the large majority of incompatible tubes were inhibited from the beginning of growth, a few grew as rapidly as the fastest compatible tubes, and penetrated into the ovary. The behavior found for pollen and pollen-tubes after crosspollination also was an approximately equal to that ofBrassica chinenensis. The longest tubes reached the ovary in about 24 hours after pollination. Brassica japonica SIEBOLD var.indivisa MAKINO Pollen behavior after incompatible selfing was found to be similar to that ofBrassica japonica. A cross-incompatible mating showed decreased germination of pollen grains and failure of pollen-tubes to penetrate into the stigma as in the incompatible selfing. On the other hand, the behavior found for pollen and pollen-tubes after cross-compatible mating was proved to be analogous to that ofBrassica japonica. The longest tubes penetrated into the ovary in about 10 hours after pollination. It may be concluded from the accounts given above that in all these Brassica species self-incompatibility is attributed to decreased germination of incompatible pollen and to no penetration of tubes into the stigma.