Yeast has 5 families of double-stranded RNAs (dsRNAs) replicating in its cytoplasm. These are called L-A, L-BC, M, T, and W. L-A, L-BC, and M are found in intracellular, noninfectious virus-like particles (VLPs). M encodes a secreted toxin and immunity to that toxin, while L-A encodes the major protein of the VLPs in which both itself and M are encapsidated. We have established an in vitro replication system for L-A, L-BC, and M. We find that (-) strand L-A synthesis occurs in VLPs carrying only the (+) strand, while (+) strand synthesis occurs by a conservative mechanism in particles carrying L-A dsRNA. New (+) strands are extruded from the VLPs. M replication occurs in vitro by a mechanism similar to that of L-A except that, because M is less than half the size of L-A, the same particles that can only hold one L-A can hold one or two M molecules. (D), a new cytoplasmic genetic element we discovered, exacerbates the disease produced by the derepression of M dsRNA replication seen in ski- mutants. We have isolated chromosomal mutants unable to maintain (D) (mad-). The MAK genes are chromosomal genes essential for M dsRNA replication, while the SKI genes are repressors of this process. We have isolated clones of MKT1, MAK11, MAK16, MAK18, SK13, SK18, and CDC16. We have completely sequenced the MAK11, MAK16, and CDC16 genes.