A study of flora and population changes in a sample paddy field in Kalat (Mashhad) were done from May to September 2007 in four stages. Physical and chemical factors such as EC, pH and temperature were measured. Totally, 23 species, 8 genera and 5 families were identified. The identified species were: Chroococcaceae: Chroococcus, Gloeothece, Aphanothece Oscillatoriaceae: Oscillatoria Rivulariaceae: Calothrix Nostocaceae: Cylindrospermum, Nostoc, Anabaena Scytonemataceae: Scytonema. Regarding the heterocystous species, the most abundant cyanobacteria belong to genus Nostoc. pH in 4 collection were alkaline and did not show significant difference. Colony number did not indicate direct relation with pH despite of EC. Maximum colony number was seen in minimum temperature.

Keywords: Cyanobacteria, Environmental factors, Nostoc, Paddy field

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