ABSTRAK Telah dilakukan penelitian mengenai struktur komunitas perifiton pada makroalga Ulva lactuca di perairan pantai Ulee Lheue, Banda Aceh. Penelitian ini bertujuan untuk mengetahui struktur komunitas perifiton yang terdiri dari kelimpahan, indeks keanekaragaman, indeks keseragaman, dan indeks dominansi, serta menghitung luas tutupan makroalga Ulva lactuca. Penelitian ini dilakukan pada bulan Februari-Maret 2016. Hasil penelitian menunjukan bahwa perifiton pada makroalga Ulva lactuca terdiri dari kelas Bacillariophyceae (13 spesies), kelas Chrysophyceae (1 spesies), kelas Cyanophyceae (3 spesies), kelas Dinophyceae (1 spesies), kelas Gastropoda (1 spesies), kelas Malacostraca (4 spesies), kelas Maxillopoda (1 spesies), kelas Pyramimonadophyceae (1 spesies), dan kelas Ophiuroidea (1 spesies). Kelimpahan perifiton yaitu 51.390-139.220 ind/cm2, dimana setiap titik pengamatan memiliki tutupan makroalga Ulva lactuca yang tergolong kategori sangat baik. Kelas Bacillariophyceae mendominasi di setiap titik pengamatan dengan jumlah rata-rata 56.809 ind/cm2. Indeks keanekaragaman perifiton berkisar 1,87-2,36 tergolong kategori sedang, indeks keseragaman berkisar 0,70-0,86 tergolong kategori tinggi, sedangkan indeks dominansi berkisar 0,12-0,23 tergolong kategori rendah. Berdasarkan nilai yang diperoleh, struktur komunitas perifiton pada makroalga Ulva lactuca di perairan pantai Ulee Lheue menunjukan tekanan ekologi yang sedang dengan tingkat dominansi rendah dan komunitas berada dalam kondisi stabil. Kata Kunci : Indeks keanekaragaman, indeks keseragaman, indeks dominansi, kelimpahan, perifiton, makroalga Ulva lactuca, Pantai Ulee Lheue.   ABSTRACT The research about community structure of periphyton on macroalgae Ulva lactuca has been done in coastal waters Ulee Lheue, Banda Aceh. The purpose of this research was to know the periphyton community structure consisting of abundance, diversity index, evenness index, and dominance index, and calculate the area covered macroalgae Ulva lactuca. The research has been started from February until March 2016. The results showed that perifiton on macroalgae Ulva lactuca consists of classes Bacillariophyceae (13 species), class Chrysophyceae (1 species), class Cyanophyceae (3 species), class Dinophyceae (1 species), class Gastropoda (1 species), class Malacostraca (4 species), class Maxillopoda (2 species), class Pyramimonadophyceae (1 species), and class Ophiuroidea (1 species). Abundance of perifiton is 51.390-139.220 ind/cm2 where each observation site has a cover of macroalgae Ulva lactuca were classified as very good category. Bacillariophyceae class dominated in every site with coverage 56.809 ind/cm2. Perifiton diversity index ranged from 1,87 to 2,36 were classified category, evenness index ranged from 0,70 to 0,86 relatively high category, while the dominance index ranged from 0,12 to 0,23 relatively low category. Based on the values obtained, the community structure of periphyton on macroalgae Ulva lactuca in coastal waters Ulee Lheue ecological pressure indicates a moderately low level of dominance and the community was in stable condition. Keywords : Abundance, diversity index, evenness index, dominance index, periphyton, macroalgae Ulva lactuca, coastal waters Ulee Lheue.