Brassinosteroids (BRs) are plant steroidal hormones that regulate plant growth and development. An Arabidopsis dwarf mutant, shrink1-D (shk1-D), was isolated and the phenotype was shown to be caused by activation of the CYP72C1 gene. CYP72C1 is a member of the cytochrome P450 monooxygenase gene family similar to BAS1/CYP734A1 that regulates BR inactivation. shk1-D has short hypocotyls in both light and dark, and short petioles and siliques. The seeds are also shortened along the longitudinal axis indicating CYP72C1 controls cell elongation. The expression of CPD, TCH4 and BAS1 were altered in CYP72C1 overexpression transgenic lines and endogenous levels of castasterone, 6-deoxocastasterone and 6-deoxotyphasterol were also altered. Unlike BAS1/CYP734A1 the expression of CYP72C1 was not changed by application of exogenous brassinolide. We propose that CYP72C1 controls BR homeostasis by modulating the concentration of BRs.