Sphingoid long-chain base (LCB) kinase catalyzes the phosphorylation of LCBs to form LCB 1-phosphates. Based on sequence identity to murine sphingosine kinase (mSPHK1), we cloned and characterized the first plant LCB kinase gene from Arabidopsis (AtLCBK1). Using recombinant AtLCBK1 protein from Escherichia coli cells, we confirmed that the enzyme specifically phosphorylated D-erythro-dihydrosphingosine (DHS), but not N-acetyl-DHS or D-threo-DHS. AtLCBK1 also phosphorylated D-erythro-sphingosine, trans-4, trans-8-sphingadienine and phytosphingosine. We found that AtLCBK1 mRNA is highly expressed in flowers. AtLCBK1 transcripts were slightly increased by low humidity or abscisic acid treatments, suggesting that AtLCBK1 is constitutively expressed under these treatments.