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Double-spikes <i>Phalaenopsis</i> orchids have greater market value than those with single-spike. In this study, a gene designated as <i>Spike Activator 1</i> (<i>SPK1</i>), which encodes a basic helix-loop-helix (bHLH) transcription factor, was isolated and characterized from <i>Phalaenopsis aphrodite</i> (moth orchid). <i>SPK1</i> was highly expressed in the meristematic tissues. In the axillary bud, <i>SPK1</i> was highly upregulated by a moderately low temperature of 20 &#176;C but downregulated by a spike inhibition temperature of 30 &#176;C. SPK1 protein is localized in the nucleus. Another bHLH, <i>bHLH35</i>, which is also highly expressed in young tissues in the same way as <i>SPK1</i> was also identified. In contrast to <i>SPK1</i>, <i>bHLH35</i> transcripts are downregulated at 20 &#176;C but upregulated at 30 &#176;C. Bimolecular florescence complementation assay and yeast two-hybrid assays indicated that SPK1 interacts with bHLH35 and forms a heterodimer. Virus-induced gene silencing (VIGS) showed that 7 out of 15 vector control plants produced double spikes but that only 1 out of 15 VIGS-<i>spk1</i> plants produced double spikes. RT-qPCR results indicated that VIGS-<i>spk1</i> downregulated gene expression levels of <i>SPK1</i>, <i>FT</i>, <i>CYCB</i>, and <i>EXPA8</i>. Overall, we propose that SPK1 plays an essential role in early axillary bud development and spike initiation of <i>P. aphrodite</i>.