In crossed electric and magnetic dc fields, gyrating electrons are subject to alternate acceleration and deceleration by the electric field. This speed modulation occurs with the gyrofrequency and periodically changes the colliding electron's velocity relative to the background neutral particles. Thus these orbiting electrons see the collision cross‐sectional area, associated with target neutral particles, as if it were varying with the gyrofrequency. In very weakly ionized gases, this effect of time‐varying collision probability can cause deviated random walk of guiding centers in the direction of the electric field applied perpendicular to the dc magnetic field. It is proposed and argued that the conventional Chapman‐Cowling type formula giving the Pedersen mobility need be reexamined in the light of this new effect. Results of computer simulation indicate that the value of the Pedersen mobility of electrons differs considerably from that given by the conventional formula.