The shoot apical meristem (SAM) is responsible for forming most of the above-ground portion of the plant. We sought to isolate regulatory genes expressed in the Arabidopsis SAM by screening a Brassica oleracea (cauliflower) meristem cDNA library with the homeobox fragment from the maize Knotted-1 (Kn1) gene. We isolated and characterized the corresponding clone, Merihb1, from Arabidopsis. Analysis shows that the predicted MERIHB1 protein exhibits strong homology to KN1 and RS1 from maize, SBH1 from soybean, and KNAT1 and KNAT2 from Arabidopsis. Merihb1 is highly expressed in mRNA from cauliflower meristems and also accumulates in stem and flower mRNA. Based on the similarity of the Merihb1 and Kn1 sequences, expression patterns, and in situ hybridizations, we suggest that Merihb1 represents an Arabidopsis homologue of the maize Kn1 gene.