Enzymes that modify cell wall components most likely play critical roles in altering size, shape, and physical properties of plant cells. Regulation of such modifying activity is expected to be important during morphogenesis and in eliciting developmental and physiological alterations that arise in response to environmental conditions. Previous work has shown that the Arabidopsis TCH4 gene encodes a xyloglucan endotransglycosylase (XET) which acts on the major hemicellulose of the plant cell wall. The expression of TCH4 is dramatically upregulated in response to several environmental stimuli (including touch, wind, darkness, heat shock, and cold shock) as well as the growth-enhancing hormones, auxin and brassinosteroids. This paper reports the presence of an extensive XET-related (XTR) gene family in Arabidopsis. In addition to TCH4, this family includes two previously identified genes, EXT and Meri-5, and at least five additional genes. The cDNAs of the XTR family share between 46 and 79% sequence identity and the predicted XTR proteins share from 37 to 84% identity. All eight proteins include potential N-terminal signal sequences and most have a conserved motif (DEIDFEFLG) that is also found in Bacillus beta-glucanase and may be important for enzyme activity. The members of the XTR gene family are differentially sensitive to environmental and hormonal stimuli. Magnitude and kinetics of regulation are distinct for the different genes. Differential regulation of expression of this complex gene family suggests a recruitment of related, yet distinct, cell wall-modifying enzymes that may control the properties of cell walls and tissues during development and in response to environmental cues.