We have characterized 46 hybrid phage which hybridize preferentially to mRNA from sporulating cells. Cross-hybridization experiments demonstrate that 27 distinct SPR (Sporulation regulated) sequences are represented among these phage. The SPR genes can be grouped into three classes: early, middle, and late. The early class shows an accumulation of transcripts soon after transfer to sporulation medium and continues to accumulate RNA throughout sporulation. Transcripts of the middle class increase in level at about the time of DNA synthesis, rise rapidly in abundance until meiosis II, then accumulate more slowly for at least the next 3 h. Late gene transcripts begin to accumulate at about the time of meiosis I, increase 10- to 20-fold in the next 2 h, then remain constant in late sporulating cells.