The phytoplankton is comprised of a diverse assemblage of organisms, which characterized by their tiny size and drifting mode. They are the first communities that respond to alterations of water quality which made them as useful indicator. The objective of this study was to determine abundance and diversity of phytoplankton in some water resources of Yazd province during four seasons. Sampling was carried out during one year period from summer 2011 to spring 2012 from ecological different locations by water collection. The physico-chemical water characteristics including; temperature, pH, electric conductivity (EC) and dissolved oxygen were measured in situ . The samples were fixed using 4 % formaline and then identification, enumeration, and composition of phytoplankton genera were carried out in laboratory. Results showed that 6 normal" Clamydomonas , and Anacystis in summer; Rhizosolenia , Nitzschia and Pleurococcus in autumn; Cyclotella , Cymbella and Gyrosigma in winter. The maximum mean density of phytoplankton was measured in genera of Bacilariophyceae (10712 cells/ml) and minimum in Euglenaophyceae (15 cells/ml). iovolume of phytoplankton was obtained according to measured size and shape. Maximum biovolume were estimated in autumn (168.3 µm 3 /ml) and minimum in winter (0.11µm 3 /ml). The highest diversity of phytoplankton was obtained in summer (6 Keywords: Phytoplankton; Abundance; Diversity; ltr"
