fa پژوهشي Orginal research papers This study evaluated the effects of diets containing 0, 2.5, 5, 7.5 and 10% <em>Arthrospira&nbsp;platensis </em>on skin pigmentation, growth performance, and digestive enzymes of koi, <em>Cyprinus carpio</em>. A completely randomized design was performed with five treatments and three replicates. One hundred and fifty koi fish with average initial weight of 30&plusmn;1g were assigned to fifteen experimental tanks. The experiment lasted for eight weeks. Koi fish fed with 2.5% <em>A. platensis</em> (1.17<span dir="RTL">&plusmn;</span>0.02) showed a significant lower growth performance than those fed with 7.5 and 10% <em>A. platensis</em> (1.39<span dir="RTL">&plusmn;</span>0.02 and 1.46) (<em>p</em><0.05). However, carotenoid (astaxanthin) concentrations of skin significantly increased with increased levels of dietary algae (<em>p</em><0.05). The highest values of a* (red zones) and b* (yellow zones) were observed in fish skin fed with 10%<em> A. platensis </em>(4.33&plusmn;0.15, 5.23&plusmn;0.15, 5.23&plusmn;0.15, 5.7&plusmn;0.36, and 6.33&plusmn;0.32, respectively). The activities of protease, amylase and lipase in dietary supplementation of 10%<em>A. platensis</em> were significantly higher than those of the control group (protease from 5.17&plusmn;0.76 to 11 , amylase from 22&plusmn;1.73 to 32.67&plusmn;1.53 and lipase from 16.33&plusmn;2.08 to 74.33&plusmn;1.53. Liver enzymes also decreased significantly by increased dietary supplementation. (ALT: from 192.33&plusmn;5.5 to 80.67&plusmn;1.52, AST: from 1741.7&plusmn;18.92 to 712.33&plusmn;10.5, ALP: from 452.33&plusmn;8.14 to 48&plusmn;1, LDH: from 7287.3&plusmn;34.64 to 7119.7&plusmn;17.89). These results revealed that inclusion of 5-10% <em>A. platensis</em> in diets has a significant positive effect on growth rate, pigmentation, and improvement of digestive and liver enzymes activities in koi fish. Arthrospira platensis, Pigmentation, Growth, Carotenoids, Enzymes activities, Koi 381 393 http://jifro.ir/browse.php?a_code=A-10-689-2&slc_lang=fa&sid=1 F. Ansarifard fansarifard@yahoo.com 100319475328460029498 100319475328460029498 No Department of Fisheries, Science and Research Branch, Islamic Azad University, P.O. Box 14515-775,Tehran, Iran. Department of Fisheries, Science and Research Branch, Islamic Azad University, P.O. Box 14515-775,Tehran, Iran. H. Rajabi Islami rajabi.h@srbiau.ac.ir 100319475328460029499 100319475328460029499 Yes Department of Fisheries, Science and Research Branch, Islamic Azad University, P.O. Box 14515-775,Tehran, Iran. Department of Fisheries, Science and Research Branch, Islamic Azad University, P.O. Box 14515-775,Tehran, Iran. M. Shamsaie Mehrjan 100319475328460029500 100319475328460029500 No Department of Fisheries, Science and Research Branch, Islamic Azad University, P.O. Box 14515-775,Tehran, Iran. Department of Fisheries, Science and Research Branch, Islamic Azad University, P.O. Box 14515-775,Tehran, Iran. M. Soltani 100319475328460029501 100319475328460029501 No Department of Aquatic Animal Health, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran. Department of Aquatic Animal Health, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran.