A total of 59 new temperature sensitive cdc mutants are described which grow normally at 25 degrees C but become blocked at DNA replication or mitosis when incubated at 36 degrees C. Thirty-nine of the mutants are altered in cdc genes which have been identified previously. The remaining 20 mutants define 10 new cdc genes. These have been characterised physiologically, and 6 of the genes (cdc 17, 20, 21, 22, 23, 24) were found to be required for DNA replication, 2 for mitosis (cdc 27, 28), and 2 (cdc 18, 19), could not be unambiguously assigned to either DNA replication or mitosis but were definitely required for one or the other. Three genes, the previously identified cdc 10, and cdc 20, 22 are likely to be required for the initiation of DNA replication. Mutants in two genes, cdc 17, 24 undergo bulk DNA synthesis at 36 degrees C, but this DNA is defective. In the case of cdc 17 the defect is in the ligation of Okazaki fragments. cdc 23 is required for bulk DNA synthesis, whilst cdc 21 may possibly be required for the initiation of a particular sub-set of replicons. A previously isolated mutant cdc 13.117 is also further described. This mutant becomes blocked in the middle of mitosis with apparently condensed chromosomes.