  We present experimental measurements of the cholesteric-smectic phase
transition of suspensions of charged semiflexible rods as a function of rod
flexibility and surface charge. The rod particles consist of the bacteriophage
M13 and closely related mutants, which are structurally identical to this
virus, but vary either in contour length and therefore ratio of persistence
length to contour length, or vary in surface charge. Surface charge is altered
in two ways; by changing solution pH and by comparing M13 with {\it fd} virus,
a mutant which differs from M13 only by the substitution of a single charged
amino acid for a neutral one per viral coat protein. Phase diagrams are
measured as a function of particle length, particle charge and ionic strength.
The experimental results are compared with existing theoretical predictions for
the phase behavior of flexible rods and charged rods. In contrast to the
isotropic-cholesteric transition, where theory and experiment agree at high
ionic strength, the nematic-smectic transition exhibits complex charge and
ionic strength dependence significantly different from predicted phase
behavior. Possible explanations for these unexpected results are discussed.
