The late genetic region of the T3 bacteriophage genome is transcribed by a phage-specified RNA polymerase, the product of T3 gene 1. In vitro, purified T3 RNA polymerase acting with T3 DNA template synthesizes six major RNA products, with molecular weights of 5.5 X 10(6), 4.2 X 10(6), 1.7 X 10(6), 0.87 X 10(6), 0.52 X 10(6), and 0.23 X 10(6). These are designated T3 RNA species I through VI, respectively. These RNAs are formed in equimolar amounts, with the exception of T3 species V, which is made in approximately twice this amount. T3 RNA species I, II, and VI have been mapped by the use of terminally deleted templates and are found to originate from promoters located at 56, 67, and 98% genome length, respectively, and to share a common terminator at approximately 100%. T3 species III, IV, and V must originate from the class III region of the T3 genome between 37.5 and 56%. Although the pattern of transcription by T3 RNA polymerase in vitro closely resembles that of the RNA polymerase from the related phage T7, neither polymerase is able to use major promoter sites for the other enzyme at an appreciable rate.