Under certain conditions two temperature-sensitive mutants of reovirus type 3 (strain Dearing) yield particles that lack either one or two of the ten genome RNA segments. They resemble the wild type strain in this respect, but there are significant differences. (i) When passaged at low multiplicity neither wild type virus nor the group F mutant ts 556 yields significant numbers of defective particles, but the group C mutant ts 447 yields progeny that consists of roughly equal amounts of nondefective particles and particles that lack genome RNA segment L3. (ii) When passaged three to four times at high multiplicity (more than 500 virus particles, equivalent to 5 to 10 PFU per cell), yields of ts 556 decrease precipitously and consist of roughly equal numbers of particles that lack RNA segment LI and particles that lack both segments L1 andL3. Only a small proportion of the yield consists of nondefective particles, (iii) Yields of wild type virus only begin to decline and contain defective particles after about eight passages at high multiplicity; progeny then consists of about one-quarter of nondefective particles and three-quarters of particles that lack segment L1. Yields of ts 447 also begin to contain particles that lack LI at this stage. All defective particles contain the normal complement of capsid polypeptides and possess full transcriptase activity. Ts 447 and ts 556 may be useful in studies on the mechanism that is responsible for selecting unique sets of single-stranded RNA molecules for encapsidation in progeny virions.