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An assessment of varying density, species composition and dynamics of phytoplankton in a combined rice-shrimp culture was carried out in My Xuyen district, Soc Trang province in the Vietnamese Mekong delta. Water was sampled six times at six separate locations within the culture system, on days 1, 20, 34, 57, 68, and day 81 of the rice-shrimp crop cycle. The results showed the abundance of 95 phytoplankton species belonging to five phyla including Cyanophyta, Chlorophyta, Bacillariophyta, Euglenophyta, and Pyrrophyta. The values of Shannon–Wiener index indicated high phytoplankton diversity, while the water quality ranged from medium to good. A Cluster Analysis showed that the phytoplankton density variation can be divided into three distinct periods. The initial phase of crop growth was dominated by <i>Pandorina morum</i>, <i>Pediastrum simplex</i>, <i>Eudorina elegans</i>, <i>Oscillatoria limosa</i>, and <i>Anabaena circinalis</i>. The midstage, reproductive phase of crop growth was dominated by <i>Scenedesnus acuminatus</i>, <i>Pediastrum duplex</i>, <i>Closterinm setaceum</i>, <i>Scenedesnus quadricauda</i>, and <i>Actinastnum hantzschii</i>. Whereas <i>Scenedesnus acuminatus</i>, <i>Scenedesnus quadricauda</i>, <i>Closterinm closterioides</i>, <i>Staurastrum arcuatum</i>, <i>Euglena nhrenbergii</i>, and <i>Phacus lnsmorensis</i> were dominant at the end of crop cycle. The findings provide useful information on phytoplankton assemblages in a typical rice-shrimp culture, which has recently gained popularity as an adaptive livelihood system.