The isolation of genes expressed during bulb formation should provide an understanding in the relationship between the induction of bulb formation and the induction of bulb dormancy. In vitro bulb formation of hyacinth was induced by low temperature and regulated by ABA as related in the preceding report. It is possible that specific genes are expressed by the increase in endogenous ABA content affected by low temperature which induces bulb formation. Thus, differential display analysis of genes induced by low temperature in bulb formation of hyacinth was conducted. Shoot explants were taken from in vitro immature leaves of hyacinth (Hyacinthus orientalis L. cv. Delft Blue). cDNAs were prepared from total RNA of the explants treated for eight weeks at 5°C or those maintained at 25°C (control), and differential display using 20 primers was performed. Cold-treated explants expressed specific fragments in eight primers; whereas, in one of the eight primers and five other primers, there were less abundant fragments in cold-treated explants than in non-cold-treated explants. It is possible that these PCR amplified fragments are related to the fluctuation of genes during the cold treatment. The PCR amplified fragments of specific expressed genes were observed in the samples just after the cold treatment. Thus it appears that the regulation of gene expression by cold treatment starts at the very early stage during or before the commencement of bulb formation.