Granite outcrops in southern Oklahoma are dotted with ephemeral rock pools, occurring in many shapes and size with varying habitat duration (hydroperiod). Pools fill during the spring and early fall, have a duration from a few days to more than a week and are home to a unique species assemblage. Organisms occupying ephemeral rock pools deal with various stresses, including desiccation and extreme daily temperature fluctuations. We have focused on collecting diatoms from rock pools across three large granite outcrops in southern Oklahoma. We will investigate the physical and chemical variables that explain their distribution. Preliminary results show that these systems are dominated by Stauroneis and Hantzschia taxa, and include species of Luticola, Achnanthes, Eunotia, Pinnularia, Muelleria, and Caloneis. Few studies have focused on microorganisms of these simple systems.This study will increase our knowledge of diatom biodiversity on granitic outcrops, and of how desiccation stress can structure natural communities.