Over the past 70 years, the distributions of the species in the genus Najas in Ohio have undergone changes. Najas gracillima and N. flexilis, native species of northern, cool, clear waters, have disappeared or become reduced in abundance, while N. marina and N. minor, European species, and N. guadalupensis, a southern native species, have invaded, spread, and/or have become more common in the state. Factors apparently responsible for these changes are (1) an increase in the numbers of artificial ponds and lakes, (2) an increase in the turbidity of Ohio waters, and (3) a gradual warming and overall general eutrophication of Ohio river and lake waters. Dated dot-distribution maps show the Ohio distributions of these species. Notes on the distribution of the species in nearby states are given.