Cyclobutane pyrimidine dimers (CPDs) constitute the most frequent UV-induced DNA photoproduct. However, it has remained unclear how human and other mammalian cells mitigate the mutagenic and carcinogenic potential of CPDs emanating from their replicative bypass. Here, we examine in human cells the roles of translesion synthesis (TLS) DNA polymerases (Pols) in the replicative bypass of a cis-syn TT dimer carried on the leading or the lagging strand DNA template in a plasmid system we have designed, and we determine in mouse cells the frequencies and mutational spectra generated from TLS occurring specifically opposite CPDs formed at TT, TC, and CC dipyrimidine sites. From these studies we draw the following conclusions: (i) TLS makes a very prominent contribution to CPD bypass on both the DNA strands during replication; (ii) Pols eta, kappa, and zeta provide alternate pathways for TLS opposite CPDs wherein Pols kappa and zeta promote mutagenic TLS opposite CPDs; and (iii) the absence of mutagenic TLS events opposite a cis-syn TT dimer in human cells and opposite CPDs formed at TT, TC, and CC sites in mouse cells that we observe upon the simultaneous knockdown of Pols kappa and zeta implicates a highly error-free role of Poleta in TLS opposite CPDs in mammalian cells. Such a remarkably high in vivo fidelity of Poleta could not have been anticipated in view of its low intrinsic fidelity. These observations have important bearing on how mammalian cells have adapted to avoid the mutagenic and carcinogenic consequences of exposure to sunlight.