Analogues of glycerol in which each of the three hydroxy groups is successively replaced by fluorine or hydrogen have been examined as substrates or inhibitors of glycerol kinase (Candida mycoderma) to assess the ability of fluorine to mimic a substrate hydroxy group in enzyme-analogue interactions. The four diols resulting from replacement of the hydroxy groups at C-1 or C-2 of sn-glycerol by fluorine or hydrogen are weak substrates. Similar substitution of the C-3 hydroxy group gives compounds which act as competitive inhibitors of glycerol or dihydroxyacetone phosphorylation but show no activity as substrates. Comparison of the steady-state kinetic parameters of the corresponding analogues shows that replacement of a hydroxy group by either fluorine or hydrogen leads to compounds with similar activity in this system. A convenient synthesis of (+)-propane-1,2-diol is described.