In the incompatibility system operating in Lythrum salicaria, the three incompatibility types Long, Mid and Short will be equally frequent in a population of infinite size. But in small populations the complete system cannot be maintained indefinitely because of random loss of incompatibility types. A simulation model for such populations shows that this may be an important factor for populations consisting of less than 20 plants. When loss actually occurs, it will in most cases involve the Short type and in some cases the Mid type, but only rarely the Long type. This result may explain why the Long type appears to be the most common one in nature and Short the most infrequent one.