Cytoplasmic male sterile line RC(7) of Chinese cabbage produces mature anthers without pollen. To understand the mechanisms involved, we examined the ultrastructural changes during development of the microspores. Development of microspores was not affected at the early tetrad stage. During the ring-vacuolated period, some large vacuoles appeared in the tapetum cells, making them larger, extending to the anther sac center during the monocyte period. At the same time, the tapetum degenerated as the microspores aborted, resulting in pollen-deficient anthers. As a result, the locules collapsed and the anthers shriveled. The callose was degraded in the pollen walls; abnormal deposits of electrodense material gave rise to irregular spike-shaped structures, rather than the characteristic rod-like shape of the B7 bacula. The internal intine wall of RC(7) was thinner than that of the B7 type. At the mitosis I microspore stage, the tapetum cells contained multiple plastids, with numerous small spherical plastoglobuli, and lipid bodies. Based on these observations, we suggest that RC(7) abortion may be due to mutated genes that normally regulate development of the pollen wall and cell walls in the RC(7) line.