Transcription of soybean retrotransposon SORE-1 was temporally upregulated during ovule development. This transcriptional pattern was under intrinsic control conferred by the long terminal repeat of SORE-1. Transcriptionally active retrotransposons are capable of inducing random disruption of genes, providing a powerful tool for mutagenesis. Activation of retrotransposons in reproductive cells, in particular, can lead to heritable changes. Here, we examined developmental control of transcription of soybean retrotransposon SORE-1. Transgenic Arabidopsis plants that contain β-glucuronidase (GUS) reporter gene fused with the SORE-1 long terminal repeat (LTR) had GUS staining in the ovule. Quantitative analysis of transcripts in plants with this DNA construct and those with the full-length SORE-1 element indicated a temporal upregulation of SORE-1 transcription during ovule development. A comparable phenomenon was also observed in soybean plants that had a recent insertion of this element in the GmphyA2 gene. These results provide evidence that the temporal upregulation of SORE-1 in the reproductive organ is sufficiently controlled by its LTR and indicate that the intrinsic expression pattern of SORE-1 is consistent with its mutagenic property.