To investigate the involvement of the putative sensor histidine kinase CKI1 in Arabidopsis thaliana development, we employed an approach that relies on the phenotype analysis of loss-of-function mutants. We report the identification of an allele of CKI1, cki1-i, which resulted from the insertion of an En-1 transposon into CKI1, and show that the mutation causes a block in embryo sac formation. We show that CKI1 is expressed throughout embryo sac development, and is switched on transiently during early seed development. These results demonstrate that gametophytic expression of a sensor-like molecule is essential for specific processes during megagametogenesis in Arabidopsis thaliana.