An Arabidopsis fatty acid elongase gene,KCS1, with a high degree of sequence identity toFAE1, encodes a 3-ketoacyl-CoA synthase which is involved in very long chain fatty acid synthesis in vegetative tissues, and which also plays a role in wax biosynthesis. Sequence analysis ofKCS1predicted that this synthase was anchored to a membrane by two adjacent N-terminal, membrane-spanning domains. Analysis of a T-DNA taggedkcs1–1mutant demonstrated the involvement of theKCS1in wax biosynthesis. Phenotypic changes in thekcs1–1mutant included thinner stems and less resistance to low humidity stress at a young age. Complete loss ofKCS1expression resulted in decreases of up to 80% in the levels of C26 to C30 wax alcohols and aldehydes, but much smaller effects were observed on the major wax components, i.e. the C29 alkanes and C29 ketones on leaves, stems and siliques. In no case did the loss ofKCS1expression result in complete loss of any individual wax component or significantly decrease the total wax load. This indicated that there was redundancy in the elongase KCS activities involved in wax synthesis. Furthermore, since alcohol, aldehyde, alkane and ketone levels were affected to varying degrees, involvement of theKCS1synthase in both the decarbonylation and acyl-reduction wax synthesis pathways was demonstrated.