Seasonal changes in the quantity and quality of phytoplankton were studied in six channel catfish culture ponds. Chlorophyll a concentrations were generally highest in the summer (averaging >200 μg 1−1) but the highest individual chlorophyll a value recorded (910 μg 1−1) occurred in the winter during a bloom of Dictyosphaerium pulchellum. On the average, green algae (Chlorophyta) and euglenoids (Euglenophyta) represented relatively constant proportions of the phytoplankton community seasonally (about 35 and 10%, respectively). In the summer and fall, blue-green algae (Cyanophyta) became abundant. Diatoms were relatively abundant at all times and constituted the majority of the community in the winter and spring.