Expansins are a family of proteins capable of inducing stress relaxation of isolated cell walls. In earlier studies, we showed that the expression of expansin genes in deepwater rice (Oryza sativaL.) is regulated by developmental, hormonal and environmental stimuli. Here, we describe the spatial distribution pattern of expansin transcripts and proteins in tissues and organs of deepwater rice usingin situmRNA hybridization and immunohistochemical analysis. Expansin transcripts and proteins are present at high levels in the growing internodal epidermis, which has thick cell walls and acts, therefore, as a growth-limiting cell layer. Expansins are also concentrated in the differentiating vascular bundles of internodes. In the primary root, expansins are predominantly expressed in the tip region, particularly in the epidermis, differentiating vascular cylinder, and around the pericyle. Developing adventitious roots and lateral root primordia also contain high levels of expansin mRNA. In the shoot apex, expansin transcripts are abundant in the emerging leaf primordia. Our results indicate that expansins play an important role in the expansion and differentiation of plant tissues and organs.