  We report measurements of the cholesteric pitch and twist elastic constant
$(K_{22})$ in monodisperse suspensions of the rod-like virus filamentous
bacteriophage {\it fd}. Measurements were taken for concentrations spanning the
entire cholesteric region at several ionic strengths and temperatures. In the
limit of high ionic strength the cholesteric pitch $(P_0)$ scales with
concentration (c) as $P_0 \propto c^{-1.66}$. As the ionic strength decreases
the scaling exponent systematically changes to lower values.
