Azolla is a floating fern, which contains the nitrogen-fixing cyanobacterium Anabaena azollae in the dorsal lobe cavity of the leaves. This study investigated the effects of seasonal changes on the pigment composition of Azolla filiculoides in a small pond located in Istanbul, Turkey. Sampling was conducted in March, April, May, October, November, December 2007. The average total chlorophyll and carotenoid content of the leaves was almost a half fold lower in March-April-May than in October-November-December. For late winter–spring, while the average chlorophyll a/b ratio of the leaves was 2.6, for autumn - early winter it was 5.5. In March, leaf dimensions reduced, whereas the anthocyanin accumulation in the leaves increased. In May, together with the increasing temperature values, while the leaf dimensions and biomass increased, the leaves returned to green. From October to November, both leaf dimensions and amount of anthocyanin in the leaves reduced. Results showed that the probable growth season of the A. filiculoides began between April and May.