Treatment of Pseudomonas phaseolicola double-stranded RNA bacteriophage Ø6 with sodium deoxycholate converted the virions to nucleocapsids, which had in vitro RNA polymerase activity. The incorporation of [3H]UMP continued for at least 7 h. The initial incorporation was detected as intermediate RNA. Radioactivity was chased first into three segments of double-stranded RNA, and then into small, medium, and large species of single-stranded RNA successively via the intermediate RNA. Several copies of single-stranded RNA at least were synthesized from a template. The RNA synthesis clearly took place by a semi-conservative mechanism with respect to templates. That is, 5-bromo UTP was incorporated into one strand of double-stranded RNA to make a hybrid RNA of brominated and unbrominated strands. Furthermore, one strand of the 3H-labeled parental double-stranded RNA was shown to be released as single-stranded RNA.