  Using the results of a high mass resolution ($256^{3}$ particles) N-body
simulation of a cluster-forming region, we study the statistic $\sigma_{v}-M$
for low mass halos (${\rm M} < 10^{12} {\rm M}_{\sun}$), and we compare it with
three theoretical models. At the final redshift halos are well described by the
Truncated Isothermal Sphere model recently introduced by Shapiro, Iliev & Raga
(1999). We speculate that this is a consequence of the action of tidal fields
on galaxy formation within clusters.
