There are five families of double-stranded RNA (dsRNA) in strains of Saccharomyces cerevisiae, called L-A, L-BC, M, T, and W. Of these, L-A, L-BC, and M are found in intracellular virus-like particles (VLPs). Their replication is controlled by over 40 chromosomal genes; some (called MAK genes) promote dsRNA replication or maintenance, others (called SKI genes) negatively control dsRNA replication. Extensive genetic interactions among the dsRNAs and the chromosomal genes are known. The VLPs containing dsRNA produce a message (+) strand RNA copy in vitro, while the VLPs containing a (+) strand synthesize a (-) strand copy to make dsRNA. The genes MAK10 and PET18 (= MAK31 + MAK32) are necessary for the structural stability of L-A dsRNA-containing particles, but not of those containing L-A (+) strand RNA. The M1 VLPs can have either one or two M1 dsRNA molecules per particle, a fact that we explain by a sort of "head-full" hypothesis. [D] (for disease) is a new cytoplasmic genetic element which, when introduced into a ski M1 strain, makes the strain unable to grow at 20 degrees C or at 37 degrees C. [D] is not located on L-A, L-BC, M, or W dsRNA. Element [D] is heat-curable, and chromosomal mutants unable to maintain [D] (mad-) have been isolated. They can maintain M1 and L-A. [B] is a cytoplasmic genetic element which suppresses the usual need of M1 for MAK11 and several other MAK genes. Element [B] is not located on L-A or M and is distinct from [D].