The study determined the composition, abundance, and distribution of phytoplankton in Lake Mainit. Plankton samples were collected in eight sampling stations four times in 2018. A total of 26 phytoplankton taxa in four groups – Bacillariophyceae/Myxophyceae, Chlorophyceae, Cyanophyceae and Euglenophyceae were identified. Aulacoseira was ranked as the most abundant taxa, followed by Zygnema, Fragilaria, Trichodesmium, and Ulothrix. ANOVA revealed no significant (P < 0.05) difference in the number of phytoplankton between the eight stations during the first sampling. In the second sampling, Bunga showed a significantly (P < 0.05) lower number of phytoplankton than the other stations. The third sampling also showed no significant (P < 0.05) difference in the number of phytoplankton between stations. During the fourth sampling, Tagbuyawan had by far (P < 0.05) the highest number of phytoplankton. Species diversity (H’) of phytoplankton was stable at Magticao, Jaliobong, Dinarawan, Kalinawan, and Bunga. The dominance of some species (Aulacoseira, Fragilaria, and Zygnema) affected the diversity and distribution of plankton. Spectrophotometric readings recorded an average Chl-a of 0.75 µg/L. The lake was still considered oligotrophic. Remediation measures and further studies on the lake towards the primary variation of plankton are recommended. Management efforts should be strengthened to support the sustainability of life in the lake.