The diel fluctuations in water temperature, percentage oxygen saturation, pH and chlorophyll a content in water under the covers of two aquatic plants, Elodea nuttallii (Planch.) St John and Trapa sp., were observed at once through one clear summer day in a shallow pond, Ojaga-ike, Chiba Pref. Marked stratifications in both water temperature and percentage oxygen saturation were established during a day under these two plant covers . There distinctly occurred trophogenic zone that resulted in oxygen supersaturation (from surface to ca. 0.5 m deep) and tropholytic zone that resulted in oxygen depletion (from ca. 1.5 m deep to bottom). The difference of the magnitude in diel fluctuation of percentage oxygen saturation near water surface between these two plant covers is finally discussed in terms of structure and function of the aquatic plant covers.