The function of a putative glycosyltransferase (At2g35100) was investigated in Arabidopsis (Arabidopsis thaliana). The protein is predicted to be a type 2 membrane protein with a signal anchor. Two independent mutant lines with T-DNA insertion in the ARABINAN DEFICIENT 1 (ARAD1) gene were analyzed. The gene was shown to be expressed in all tissues but particularly in vascular tissues of leaves and stems. Analysis of cell wall polysaccharides isolated from leaves and stems showed that arabinose content was reduced to about 75% and 46%, respectively, of wild-type levels. Immunohistochemical analysis indicated a specific decrease in arabinan with no change in other pectic domains or in glycoproteins. The cellular structure of the stem was also not altered. Isolated rhamnogalacturonan I from mutant tissues contained only about 30% of the wild-type amount of arabinose, confirming the specific deficiency in arabinan. Linkage analysis showed that the small amount of arabinan present in mutant tissue was structurally similar to that of the wild type. Transformation of mutant plants with the ARAD1 gene driven by the 35S promoter led to full complementation of the phenotype, but none of the transformants had more arabinan than the wild-type level. The data suggest that ARAD1 is an arabinan alpha-1,5-arabinosyltransferase. To our knowledge, the identification of other L-arabinosyltransferases has not been published.