  We discuss the properties of the X-ray sources with faint optical
counterparts in the very young open cluster NGC 6231. From their positions in
the H-R diagram, we find that the bulk of these objects probably consists of
low-mass pre-main sequence stars with masses in the range 0.3 to 3.0
M$_{\odot}$. The age distribution of these objects indicates that low-mass star
formation in NGC 6231 started more than 10 Myr ago and culminated in a
starburst-like event about 1 to 4 Myr ago when the bulk of the low-mass PMS
stars as well as the massive cluster members formed. We find no evidence for a
spatial age gradient that could point towards a sequential star formation
process. Only a few X-ray sources have counterparts with a reddening exceeding
the average value of the cluster or with infrared colours indicating the
presence of a moderate near-IR excess. The X-ray spectra of the brightest PMS
sources are best fitted by rather hard thermal plasma models and a significant
fraction of these sources display flares in their light curve. The X-ray
brightest flaring sources have decay times between 2 and 16 ks. The X-ray
selected PMS stars in NGC 6231 have $\log{L_{\rm X}/L_{\rm bol}}$ values that
increase strongly with decreasing bolometric luminosity and can reach a
saturation level ($\log{L_{\rm X}/L_{\rm bol}} \sim -2.4$) for non-flaring
sources and even more extreme values during flares.

Astrophysics