ABSTRAK Tujuan penelitian ini adalah untuk melihat struktur komunitas fitoplankton di perairan Ujung Pancu berdasarkan kelimpahan, keanekaragaman, keseragaman, dominansi dan indeks morisita fitoplankton. Penelitian ini dilakukan pada bulan Desember 2017 – Januari 2018. Pengambilan sampel dilakukan dengan metode purpossive sampling. Sampel dianalisis di Laboratorium BPAP Ujung Batee dan Laboratorium Biologi Laut, Fakultas Kelautan dan Perikanan, Universitas Syiah Kuala. Hasil penelitian menunjukkan jenis fitoplankton yang paling banyak ditemukan adalah jenis Chaetoceros sp. yang berasal dari kelas Bacillariophyceae (diatom) dengan nilai kelimpahan 0,469 ind/ml. Nilai keanekaragaman fitoplankton diperoleh berkisar antara 1,52 – 2,29. Nilai keseragaman fitoplankton berkisar antara 0,69 – 0,92. Nilai dominansi berkisar antara 0,12 – 0,33 dan nilai indeks morisita berkisar antara -2,09 – 0,39. Berdasarkan nilai kelimpahan dan keanekaragamannya, dapat dinyatakan bahwa struktur komunitas dan penyebaran fitoplankton di perairan Ujung Pancu tergolong dalam kondisi stabil. Kata kunci : Stuktur Komunitas; fitoplankton; Perairan Ujung Pancu Aceh Besar. ABSTRACT The objective of this study was to figure out the community structure of phytoplankton in waters based on its abundance, diversity, evenness, dominant and morisity index. The research was conducted from December 2017 to January 2018. Sampling stations ware determined using purpossive sampling method. Samples were analyzed at laboratory of BBAP Ujung Batee and at Marine Biology Laboratory, Marine and Fisheries Faculty, Syiah Kuala University. The results showed that the most common type of phytoplankton was Chaetoceros sp. derived from the Bacillariophyceae class (diatoms) with an abundance value of 0.469 ind/ml. The diversity value of phytoplankton ranged from 1.52 to 2.29. The value of phytoplankton diversity obtained ranged from 1.52 to 2.29. The evenness value ranged from 0.69 to 0.92. Dominant value ranged from 0.12 to 0.33 and the index value of morisity index ranged from -2.09 to 0.39 Based on the value of community structure and distribution of phytoplankton the water of ujung pancu were in stable condition. Keywords: community structure; phytoplankton; Ujung Pancu Aceh Besar