A number of temperature-sensitive mutants of vaccinia virus strain WR have been isolated. Some are more heat labile than wild-type virus and are probably capsid protein mutants. One mutant of a different type, ts 20.165, was investigated in some detail. This mutant has a delayed and decreased rate of DNA replication, as well as a diminished rate of early messenger RNA transcription at the nonpermissive temperature (39°). However, both of these defects, and especially the latter, also operate at the permissive temperature (31°), at which the viral yield is as high as that of wild-type virus. Late functions, such as the transcription of late messenger RNA and switch-off of the synthesis of DNA polymerase, are normal. Temperature-shift experiments confirmed that ts 20.165 is an early function mutant. ts 20.165 virions have associated with them less than one-half of the amount of RNA polymerase associated with wild-type virions. Further, the DNA polymerase coded by the mutant is about twice as heat labile as the enzyme coded by wild-type virus. ts 20.165 virus particles are formed at 39°. These particles have only about 3% of the specific infectivity of particles formed at 31°. They contain, as far as one can tell, a normal amount of DNA and a normal capsid protein complement, and are normal in appearance. However, the yield at 39° has a higher frequency of revertants, on a PFU basis, than the yield at 31°. ts 20.165 recombines with high efficiency with two other ts mutants of vaccinia WR. Complementation is weak, but detectable. It is concluded that ts 20.165 is an early function mutant. The possible nature of its defect is discussed.