The study was carried out to quantify the periphyton biomass developed on glass substrates over time, to investigate the effects of periphyton quantity and fish size on the ingestion rate by fish, and to determine the feed conversion ratio (FCR) of periphyton by tilapia Oreochromis niloticus. Periphyton was grown in two fertilised 1000-l tanks on glass slides and monitored as dry matter (g), ash-free dry matter (g) and chlorophyll a concentrations (mg) per unit surface area (m2) over a six week period. Ingestion rate was determined for two sizes of tilapia (7 and 24 g) which were provided with four different periphyton densities. Determination of FCR was made after feeding three individual fish ad libitum with periphyton for two weeks. Periphyton ash-free dry matter increased sharply during the first half of the trial with a peak being recorded at week 3 (75.5 g m-2). Productivity was 2.4 g ash-free dry matter m-2 d-1 during the first three weeks. Mean chlorophyll a concentration showed a cyclic pattern throughout the study with the lowest value being measured during the last week. Ingestion rates were 0.90 and 0.18 mg dry matter g fish body weight-1 h-1 for small and medium fish respectively. Ingestion rate among small fish increased significantly (P <0.05) with periphyton density, but not for medium size fish. Although periphyton ash content was high (55% dry matter), fish growth was sustained. Fish harvested 70% of total periphyton dry matter that was offered to them. The FCR for periphyton was 2.81 on a dry matter basis and 1.34 on an ash-free dry matter basis.