A fine, unligated clot of human or bovine fibrin prepared from purified fibrinogen is subjected to a large torsional deformation (maximum shear strain γ up to 1.37) with superposed small oscillating deformations (Δγ ca. 0.03) at frequencies from 0.2 to 1 Hz. The “secant modulus” is defined asGi(γ)=σi/γ,where σ is stress and the subscriptirefers to an initial measurement about 25 s after imposition of strain. The storage modulusG′(ω,γ)refers to a differential oscillating measurement at frequency ω superposed on a static strain γ. For γ up to about 0.1,Gi(γ)was independent of γ and equal toG′(ω,0)measured at about 1 Hz and zero static strain. At large static deformations, the differential storage modulusG′(ω,γ)could be used to monitor changes in structure. Since there is very little time dependence of the relaxation modulus in the range from 1 to 60 s, and the loss tangent is very small,Gi′(ω,γ)could be considered simply as the differential modulusGi(γ)+γdGi/dγ,and agreed with the latter expression as long as both measurements(Gi,G1′)were made within a short time lapse. At high static strains, the initial differential modulusGi′(ω,γ)was larger than the corresponding zero‐static‐strain valueG′(ω,0)by as much as a factor of 60. During a stress relaxation experiment at large constant strain,G′(ω,γ;t)decreased with elapsed timetbut relatively less than the nonlinear relaxation modulusG(γ;t).After a clot had been subjected to a large strain for a long time and then released,G′(ω,γ;t)fell at once to a value much less thanG′(ω,0)but slowly increased toward the latter value. During a creep experiment at large constant stress,G′(ω,γ;t)increased but much less than if it were governed by the strain magnitude alone. Ligation did not affect the dependence ofG′on γ but largely eliminated its dependence on elapsed time in experiments of stress relaxation and subsequent release. The behavior can be interpreted qualitatively in terms of three processes: enhancement of structure at large strains (perhaps associated with an increased number of contact points between protofibrils), release of the enhancement (perhaps due to rupture of protofibrils), and subsequent healing of rupture by reassociation of noncovalent bonding sites.