We present two experiments that probe the temperature dependence of th
e flexibility of the filamentous virus fd by examining aspects of the
liquid crystalline nature of fd suspensions. The first measurement is
of the temperature variation of the coexisting fd concentrations at th
e isotropic-cholesteric phase transition. The second measurement is of
the magnetic field induced birefringence or Cotton-Mouton constant in
the isotropic phase as a function of temperature. We compare these me
asurements with the theoretical treatment of Onsager, Khokhlov-Semenov
, and Chen and conclude that the flexibility of fd varies nonmonotonic
ally with temperature and has a minimum in persistence length at 35 de
grees C. (C) 1996 John Wiley & Sons, Inc.