[en] A sequential model of multiple vacancy production according to which the vacancies are created in successive collisions of a fast ion with electrons belonging to a definite shell of an atom, with different single electron ionization probabilities, is discussed. A simple formula for the probability distribution of the multiple vacancy production is derived in the framework of this model which must be valid for sufficiently highly charged ions. Comparison of this formula with experiments shows that the sequential model allows discrepancies between the widely used binomial distribution and these experiments to be explained without invoking any fitted parameters. (Author)