A recent paper (Imazu, 1986) purports to demonstrate that the fusion reaction rates in high temperature plasmas decrease as the strength of the applied magnetic field rises. The magnitudes of the claimed effect is significant, the B=5T rate being only half the B=0 rate. If one leaves aside the question of the reacting particles being spin polarized, then it is well known (Jancel and Kahan, 1966) that a magnetic field has no effect whatever on the velocity distribution of particles in thermal equilibrium, hence it can have no effect on the fusion reaction rate. The error in the paper by Imazu can be found in equation 7 where a geometrical model of the particle orbits has been misinterpreted.