Organised cell division and expansion play important roles in plant embryogenesis. To address their cellular basis, we have analysed Arabidopsis abnormal-embryo mutants which were isolated for their characteristic phenotype: mutant embryos are small, mushroom-shaped ("pilz") and consist of only one or few large cells each containing one or more variably enlarged nuclei and often cell wall stubs. These 23 mutants represent four genes, PFIFFERLING, HALLIMASCH, CHAMPIGNON, and PORCINO, which map to different chromosomes. All four genes have very similar mutant phenotypes although porcino embryos often consisted of only one large cell, The endosperm did not cellularise and contained a variably reduced number of highly enlarged nuclei, By contrast, genetic evidence suggests that these genes are not required for gametophyte development. Expression of cell cycle genes, Cdc2a, CyclinA2 and CyclinB1, and the cytokinesis-specific KNOLLE gene was not altered in mutant embryos. However, KNOLLE syntaxin accumulated in patches but no KNOLLE-positive structure resembling a forming cell plate occurred in mitotic cells, A general defect in microtubule assembly was observed in all mutants. Interphase cells lacked cortical microtubules, and spindles were absent from mitotic nuclei although in rare cases, short stubs of microtubules were attached to partially condensed chromosomes, Our results suggest that the cellular components affected by the pilz group mutations are necessary for continuous microtubule organisation, mitotic division and cytokinesis but do not mediate cell cycle progression.