A theory is presented to explain the stretching and orientation of a semiflexible chain such as the biopolymeric filamentous actin and deoxyribonucleic acid in a magnetic field. Both dilute and concentrated solutions are considered. The theory predicts the requirement of a high magnetic field to achieve a high degree of orientation in a dilute solution. The experimental value of the order parameter for a dilute, polydisperse F-actin solution has been compared with the theoretical value. On the other hand, a molecular field (nematic liquid crystal) which exists in a concentrated solution can achieve the same effects of stretching and orientation and can be assisted by a magnetic field. Sources of diamagnetic anisotropy in biopolymers have been cited,