  Evidence for a second main-sequence turn-off in a deep colour-magnitude
diagram of NGC 1868 is presented. The data were obtained with HST/WFPC2 and
reach down to m_{555} ~ 25. Besides the usual t ~ 0.8 Gyr turn-off found in
previous analyses, another possible turn-off is seen at m_{555} ~ 21 (M_V ~
2.5), which is consistent with an age of t ~ 3 Gyrs. This CMD feature stands
out clearly especially when contaminating field LMC stars are statistically
removed. The background subtracted CMD also visibly displays a red subgiant
branch extending about 1.5 mag below the younger turn-off and the clump of red
giants. The significance of the secondary turn-off in NGC 1868 was confirmed
with Monte-Carlo simulations and bootstrapping techniques. Star-counts in
selected regions in the cluster CMD indicate a mass ratio of old
population/young population in the range 5% <= M_{old} / M_{young} <= 12%,
depending on the mass function slope. The existence of such a subpopulation in
NGC 1868 is significant even in the presence of uncertainties in background
subtraction. The possibility that the secondary turn-off is associated with the
field star population was examined by searching for similar features in CMDs of
field stars. Statistically significant excesses of stars redwards of the
main-sequence were found in all such fields in the range 20 <= m_{555} <= 22.
These however are much broader features that do not resemble the main-sequence
termination of a single population. We also discuss other alternative
explanations for the feature at m_{555} ~ 21, such as unresolved binarism,
peculiar stars or CMD discontinuities associated with the Bohm-Vitense gap.
