Messenger RNA maturation in eukaryotes typically involves the removal of introns from long precursor molecules. An unusual form of RNA splicing in which separate precursor transcripts contribute sequences to the mature mRNA through intermolecular reactions has now been documented in a number of diverse organisms. In this review, the phenomenon of pre‐mRNAtrans‐splicing has been divided into two categories. The “spliced leader” type, found in protozoans such as trypanosomes and lower invertebrates such as nematodes, results in the addition of a short, capped 5′ noncoding sequence to the mRNA. The “discontinuous group II intron” form oftrans‐splicing, found in plant/algal chloroplasts and plant mitochondria, involves the joining of independently transcribed coding sequences, presumably through interactions between “intronic” RNA pieces. Both categories oftrans‐splicing are mechanistically similar to conventional nuclear pre‐mRNAcis‐splicing; potential evolutionary relationships are discussed.— Bonen, L.Trans‐splicing of pre‐mRNA in plants, animals, and protists.FASEB J.7: 40‐46; 1993.