Frequency of occurrence changes are cited for 69 non-indigenous plant taxa listed by Edwin L. Moseley in 1928. Of these taxa, approximately 38% are more frequent now (1978) than 50 years ago. Twenty-nine percent exhibit the same frequency of occurrence, while 33% are less frequent than they were 50 years ago. The number of non-indigenous taxa in the flora has increased from 10% to 18% during the past 50 years. Changes in the frequency of occurrence generally are brought about by changes in roadside habitats, railroad maintenance activities, and by construction of new residential communities.