Spirulina platensis is an attractive human food supplement, animals feed and of numerous medicinal uses. The effect of physico-chemical factors on the physiological growth rates as optical densities (O.D.) of S. platensis strain (UTEXLB2340) were investigated. Experiments were conducted at different pH levels, light intensities, and temperature regimes, and monitored for 20 days. The half concentration of Zarrouk media was found to be suitable for cultivation of this strain under indoor (1.14

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... D.) and outdoor conditions (0.99 O.D.). The nitrogen source sodium nitrate (2.5 gl -1 ) can be replaced by inexpensive and low concentration of urea fertilizer (0.12 gl -1 ). The growth rates of this strain were optimum at light intensities of 1500 to 2500 lux of continuous light (0.99 to 1.71 O.D.) or 2500 lux of intermittent light (1.32 O.D.). The optimal growth was recorded at lower pH 7 to 8 (1.19 to 1.76 O.D.), and temperature of 25 to 35°C (1. 13 to 1.71 O.D.) . Therefore, this microalga can be cultivated for commercial uses in the available natural ponds where the average temperature reached 30°C and light intensities fluctuated between 1000 to 4000 lux in the morning, 12,000 to 22,000 lux midday, and 7000 to 10,000 lux in the noon.