It is shown that thin galaxies with an apparent axial ratio b/a less-t
han-or-equal-to 0.1 viewed edge-on have a geometrical thickness that c
an be explained only by the presence of a massive dark halo. Analysis
of the stability condition for stellar disks and the results of numeri
cal simulation of N-body galaxies lead to the conclusion that the mini
mum possible relative thickness z0/R0 of a collisionless disk decrease
s with increasing relative mass-mu of the halo. Therefore, existing me
asurements of the ratio z0/R0 for disks of normal galaxies can be used
to estimate the relative halo mass.