Abstract. – Diatom composition and abundance in biofilm that formed on artificial substrates exposed at several depths in July 2000 were investigated at two stations in Croatia’s oligotrophic and highly-stratified Zrmanja Estuary. Chlorophyll a ranged from 2.27 to 92.81 mg m-2. HPLC analysis provided evidence of diatom dominance, while green algae and cyanobacteria contributed less than 1 %. On 57 diatom species, two genera, Navicula and Nitzschia, dominated. After 2 weeks of exposure, diatom abundance was 2.3 x 107 cells cm-2 and species richness was 41; after 4 weeks, abundance doubled and richness increased to 50. Periphyton was composed mostly of Amphora coffeaeformis and Navicula veneta (≥ 91 % of samples, average abundance 106-107 cells cm-2) after 2 weeks; after 4 weeks, Melosira moniliformis was co-dominant (91 % and 2.3-8.6 x 106 cells cm-2). Maximum diatom abundance was at 0.5 m after 2 weeks and 1.5 m after 4 weeks. Shannon’s diversity index ranged between 0.87 and 2.08 at the upper estuary station and between 1.1 and 2.7 at the other. Amphora coffeaeformis and Navicula veneta contributed more than 80 % to abundance at both stations, averaging 106-1.8 x 107 cells cm-2.