Bacteriophage sk1 is a small isometric-headed lytic phage that infects Lactococcus lactis. The phage has a linear double-stranded DNA genome of 28 kbp, with cohesive ends. RNA was prepared from phage-infected L. lactis cells harvested at various intervals after infection, and the RNA molecules were resolved by electrophoresis. Northern blots of these gels were hybridized with sk1 DNA probes and the results obtained from these experiments, together with the results of primer extension analyses, enabled a transcription map of the phage genome to be prepared. Three classes of phage transcripts, designated as early, middle or late based on their time of appearance, were detected. Seven partially overlapping early transcripts were detected; these were transcribed from a 10 kbp region of the phage. The nine middle transcripts were derived from a 2 kbp region, limited by cos at one end and the start of the early transcripts at the other. The early and middle transcripts were transcribed divergently from a region mapping at 26 kbp on the sk1 physical map. The four late transcripts were derived from a 16 kbp region of the phage limited at one end by cos. The late transcripts were transcribed in the opposite direction to the early transcripts and three of the late transcripts terminated in the same region of the phage genome as three of the early transcripts.