Kulkoyluoglu, Okan, Necmettin Sari, Derya Akdemir, Mehmet Yavuzatmaca, Ceren Altinbag.. Distribution of sexual and asexual ostracoda (crustacean) from different altitudinal ranges in the Ordu region of Turkey: Testing the Rapoport Rule. High Alt. Med. Biol. 13: 126-137.-We evaluated Rapoport's rule which states a negative correlation between species richness and altitude. To understand the relationship between altitude and reproductive modes (a/sexual) of non-marine ostracods, 166 aquatic bodies in Ordu region, Turkey were randomly sampled from July 11 to July 16, 2010. Atotal of 26 species of ostracods were found from 133 out of 166 sites. Except for one species (Heterocypris incongruens), the other 25 species were new reports for the region. Candona improvisa was also a new report for Turkish ostracod fauna. Three species (Psychrodromus olivaceus, H. incongruens, and C. neglecta) occurred most frequently as 43, 46, and 76 times, respectively. Canonical correspondence analyses exhibited two variables [Habitat type (p = 0.014; F = 2.171) and water temperature (p = 0.018; F = 2.248)] as having the most effect on species. Correlation of species' reproductive modes to those of environmental variables measured was not significant. UPGMA dendrogram displayed 15 most frequently occurring species into four clusters where most species (11) were asexual. Although a small group (asexual species without swimming setae) showed a tendency to habitat type and electrical conductivity, such variables are believed to play secondary role on species distribution. Highest species diversity (13 species) was observed at the range of 1200 and 1400m (a.s.l.), where numbers of stations sampled was not the highest (22). Numbers of asexual species (19) were higher than the sexual (11) but there were no significant differences (p > 0.05) between the frequencies of their occurrences at different altitudinal ranges. Accordingly, our findings do not support the Rapoport Rule. Results yield that reproductive modes of species (sexual and asexual) was not directly correlated to altitude or any environmental variables measured during this study. A better explanation of ostracod diversity appears to be suitability of habitats.