A simulation model has been set up in order to study the population genetics of the disassortative mating system operating in Lythrum salicaria. The model makes it possible to consider the effect of a restricted population size on the rate of random loss of one of the forms Long, Mid and Short. It is found that isolated populations consisting of 20 individuals or more are generally quite stable. It is proposed that the observed different frequencies of the three forms in nature may be accounted for by unequal tendencies to disappear locally because of random effects.