A DNA sequence encompassing intervening sequence 2 of the soybean leghemoglobin gene was inserted into the cauliflower mosaic virus (CaMV) genome. Upon passage through host plants progeny virus slowly accumulated that had lost the intron, apparently by exact splicing of the RNA form of the genome at the exon/intron borders. These findings suggest that reverse transcription from (spliced) RNA to DNA is a normal step in CaMV replication. S1 nuclease mapping revealed a mixed population of unspliced and spliced viral RNAs. The inefficiency of the splicing process is discussed.