AbstractThis paper embodies findings of twelve month on the effect of straw‐board mill effluents on the distribution and diversity of phytoplankton of the river Dhasan. The phytoplankton of the three stretches (Reference Station A, Effluent Mixing Station B and Diluted Station C) of the river was sampled regularly every month and analysed qualitatively and quantitatively. Simultaneously some of the relevant chemical parameters of the water were also analysed. The result of the water were also analysed. The result of the plankton study showed that the plankton concentration was very low. It ranged from 166 organisms to 2240 organisms per litre only. The maximum and minimum numbers were recorded in February and June‐July respectively at all stations. Winter was found to be the most favourable season and the rainy season the most unfavourable with regard to the population density. From the samples collected from all the stations we could recognise 54 genera of chlorophyceae, 25 genera of Bacillariophyceae, 24 genera of cyanophyceae, 3 genera of Dinophyceae and 2 genera each of euglenophyceae, chrysophyceae and xanthophyceae. The percentage composition of the various groups varied with stations, but by and large chlorophyceae and bacillariophyceae dominated in all the sampling stations. The data were subjected to ‘ANOVA’ and ‘t’ tests. The results show that the observed variations in the distribution and diversity of phytoplankton among the three stations were not significant at the 5= level except in the case of Eug