The progression of mitotic division cycles and synchronous development between and within the male and female reproductive organs are essential for plant sexual reproduction. Little is known about the genetic control of the progression of mitotic cycles of the haploid genome during gametogenesis in higher plants. Here, we report the phenotypic and molecular characterization of an Arabidopsis thaliana mutant, slow walker1 (swa1), in which the progression of the mitotic division cycles of the female gametophyte was disrupted. Confocal microscopy revealed that megagametophyte development was asynchronous in swa1, causing embryo sacs to arrest at two-, four-, or eight-nucleate stages within the same pistil. A delayed pollination experiment showed that a small fraction of the swa1 embryo sacs were able to develop into functional female gametophytes. The swa1 mutation also showed a slight reduction in penetrance through the male gametophyte, although the pollen grains were morphologically normal. Molecular analysis indicates that SWA1 encodes a protein with six WD40 repeats that is localized in the nucleolus in interphase cells. The SWA1 gene is expressed in cells undergoing active cell divisions, including functional megaspores and the female gametophytic cells. RNA interference results indicated that knockout of SWA1 inhibited root growth significantly and led to the accumulation of unprocessed 18S pre-rRNA. These data suggest that SWA1 most likely plays a role in rRNA biogenesis that is essential for the progression of the mitotic division cycles during gametogenesis in plants.