In Caenorhabditis elegans, pre-mRNAs that are trans-spliced are distinguished by the presence of an 'outron', intron-like RNA at the 5' end followed by a splice acceptor. We report that trans-splicing of the rol-6 gene can be completely suppressed simply by introducing a donor site into its 173 nt outron, at a site 50 nt upstream of the trans-splice site, thereby converting rol-6 into a conventional gene with a spliced intron near its 5' end. When the consensus donor site was inserted at sites further upstream it was less effective in replacing transplicing with cis-splicing. Surprisingly, the length of the intron was not the important variable, since lengthening of the 50 nt intron to 250 nt did not restore trans-splicing. Apparently the context into which the splice site was introduced determined the efficiency of its use. These results support the conclusion that the sole signal for trans-splicing is the presence of an outron. Clearly, cis- and trans-splice acceptor sites are interchangeable, allowing the possibility of competition between the two types of splicing.