Martensi tic transformations in cylindrical p rotein crystals are found to perform life functions in primitive biological systems. Tail- sheath contraction in T-even bacteriophages can be described as an irre- versible strain-induced martensitic transformation, while polymorphic trans- formations in bacterial flagella appear to be stress-assisted mechanically reversible martensi tic transformations exhibiting a shape memory effect. Available information indicates that the geometric, thermodynamic, and kinetic aspects of these transformations are c onsistent with martensitic be- havior. Similar transformations involving the motion of partial (coherency) dislocations under chemical forces may underly the mechanism of motion in higher organisms as well. Introduction.- Recent advances in molecular biology have revealed that a remarkable number of biological structures i nvolve p eriodic or crystalline arrays of protein molecules. Even more remarkable, from the viewpoint of a materials scientist, is the f act that displacive phase transformations between metastable states f requently occur and provide the mechanism of important life processes. While crystal struc- tures of importance to materials science, and metallurgy in particular, have been known for quite some time and considerable attention has focussed on the mechanism and kinetics of solid-state transformations, the question of transformation mechan- isms in the recently determined molecular crystal structures is just now being addressed in the field of biology. These systems may provide important tests of the generality of fundamental concepts in materials science. We here examine the relevance of martensi tic transformation theory to biologi- cal systems adapting the t heory to reduced dimensionality using two examples of displacive transformations in cylindrical protein crystals performing