The complementation group (CG) assignment is presented for 74 moderately sensitive (∼2−4× sensitivity of parental line based on ratio of D10S) UV-sensitive mutants of Chinese hamster ovary (CHO) cells from the Facility for Automated Experiments in Cell Biology (FAECB) collection. The distribution of mutants within the first five rodent UV CGs was similar to that of previously reported highly sensitive (>4× wild-type UV sensitivity) mutants from this collection. This analysis nearly completes the identification of this large collection of over 200 mutant lines isolated after screening an estimated 3 million total colonies of mutagenized CHO cells from ∼20 mutant hunts with up to about 400 000 colonies screened. Only eight lines with less than about 2× parental line UV sensitivity remain unassigned. One CG of UV mutants (CG6), which now has five identified representatives in the collection, has only been found among moderately UV-sensitive CHO cells. Mutant UV40, a mitomycin C (MMC)- and X-ray-sensitive line with moderate UV cross-sensitivity, is not in CGs 1–6 and apparently is not a nucleotide excision repair mutant. Also identified were new alleles of CG1 and CG4 mutants with profoundly deficient unscheduled DNA synthesis and moderate UV sensitivity but low sensitivity to MMC. The first CG5 mutant derived from MMC-sensitive MC5 cells has been identified as the second CG5 mutant in the collection. No representatives of rodent CGs 7–11 were found, suggesting that AA8 cells have a chromosomal makeup that precludes easy isolation of mutants in these CGs. The only mutagenesis treatment generating mutants in all of the CGs 1–6 as well as the MMC mutant UV40 was with the frameshift mutagen ICR170. New CGs were not found using mutagen-sensitive derivatives of AA8 (EM9, MC5) in place of wild-type cells, although these parental cell lines did allow the isolation of new double mutant genotypes.