Synchrony with respect to cell division and DNA synthesis in cultures of Anacystis nidulans was induced by a light-dark-light regimen. At periodic intervals in the cell-division cycle, DNA, RNA, protein contents, UV sensitivity and induction of mutations were assayed. The DNA, RNA and protein syntheses were periodic and reached maximal values before the separation of cells. The DNA content started to increase at about the 5th hour and doubled at about the 13th hour followed by a plateau of 4-6 h. Wild-type A. nidulans was highly sensitive to UV radiation during the period showing an increase in cell number (rise phase) and the early part of DNA synthesis (synthetic phase). Significant resistance to UV, however, developed in the later stage of the DNA synthesis. This resistance decreased considerably during the next rise phase. On the other hand, in a UV-sensitive strain of A. nidulans (uvs67) there was no appreciable change in the UV sensitivity during the cell-division and DNA-synthesis phases. Induction of mutation frequency patterns of all the markers (fil, blu, yel, vir, nit, strR) in the wild-type showed a short initial lag followed by an abrupt increase resulting in a peak of mutation frequency in the early part of DNA synthesis and subsequently a second plateau. The induction of mutation frequencies in the uvs67 strain was comparatively low and remained constant throughout the cell division cycle. These results suggest the possibilities of an error-prone dark repair and a stringent relationship between DNA replication and repair of UV damage for expression of mutations in cyanobacterium A. nidulans.