We have purified and characterized a particle sedimenting at 20S from the postribosomal fraction of yeast, wheat germ, Drosophila melanogaster tissue culture cells, chicken embryo fibroblasts, rabbit reticulocyte lysate, and HeLa cells. Most of the protein constituents of the 20S particle have molecular weights of 20-35 kd and differ between species; however, some do have similar molecular weights and isoelectric points, suggesting they are related. Several low-molecular-weight RNAs, distinct from tRNAs, co-purify with the particle isolated from all these species and show increasingly more complex patterns ascending the arbitrary order from yeast to human (yeast, plant, insect, bird, and mammals). In Drosophila, we present evidence that these small RNAs are tightly associated with this 20S structure.