RNAs from the ciliates Euplotes octocarinatus, Paramecium tetraurelia, Stylonychia lemnae, Tetrahymena thermophila and the snail Lymnaea stagnalis were isolated and compared for structural differences. The organisms were found to differ considerably in the size and structure of their rRNAs, the extent of polyadenylation and the length of the respective poly A tails of their mRNAs. The mRNAs of Euplotes, Paramecium, Stylonychia and Lymnaea were also compared for in vitro translatability in the rabbit reticulocyte lysate and the wheat germ translation systems. In accordance with earlier observations for Paramecium [15] and the finding that Paramecium, Stylonychia, Oxytricha and Tetrahymena deviate from the universal genetic code by using the stop codons UAA and UAG to encode glutamine [4, 8,11,12,21] it was found that Paramecium and Stylonychia poly(A)(+) RNAs were poor templates for in vitro translation. Unexpectedly, however, poly(A)(+) RNA of the ciliate Euplotes was readily translated in vitro and yielded a great number of polypeptides with different molecular weights comparable to the data obtained with poly(A)(+) RNA of Lymnaea.