We propose a new method to measure the mass of large-scale filaments in galaxy redshift surveys .
The method is based on the fact that the mass per unit length of isothermal filaments depends only on their transverse velocity dispersion .
Filaments that lie perpendicular to the line of sight may therefore have their mass per unit length measured from their thickness in redshift space .
We present preliminary tests of the method and find that it predicts the mass per unit length of filaments in an N-body simulation to an accuracy of \sim 35 \% .
Applying the method to a select region of the Perseus-Pisces supercluster yields a mass-to-light ratio of M / L _ { B } \approx 460 h in solar units to within a factor of two .
The method measures the mass-to-light ratio on length scales of up to \sim 50 { h ^ { -1 } Mpc } and could thereby yield new information on the behavior of the dark matter on mass scales well beyond that of clusters of galaxies .